It can often be said, in the case of three languages (say X, Y and Z) of the same family, that Y is “between” X and Z. For example, as stated in Wikipedia (with several references given), “Dutch is closely related to German and English and is said to be between them.” Such “betweenness” is roughly symmetric, in the sense that Y has some features in common with X and others with Z, in addition to having some that are shared with neither. It can, more over, be refined further: Frisian can be said to be between Dutch and English, and Low German between Dutch and German. In a similar way, Catalan is between French and Spanish, while Occitan is between Catalan and French, and Aragonese is between Catalan and Spanish.
But there are some language threesomes that are asymmetric in a peculiar way: Y is like X on one level, and like Z on another. More specifically, X and Y are, historically, one and the same language, but because of a political and/or geographic separation between their territories, and a corresponding association with the territory of Z, Y sounds much more like Z than like X. Also, while the traditional vocabulary of Y is the same as that of X, its modern vocabulary is more like that of Z.
The two examples that first struck me, because of their similarities, are Portuguese-Galician-Spanish and Bulgarian-Macedonian-Serbian. The time scales of the political connections are different: Galicia has been under Spanish (Castilian) rule since the 13th century, while Macedonia (the present Republic) was under Serbian control or influence only during the 20th century. But the effects are similar.
Fundamentally, Portuguese and Galician are the same language — modern descendants of the medieval language that is now called Galician-Portuguese — with some of the same grammatical peculiarities (such as the personal infinitive) that differentiate it from other Romance languages, and in their written forms (aside from the different orthographies, to be discussed below) they look much the same.
But Galician sounds like Castilian Spanish; except for having /ʃ/ (which is spelled x) in place of /χ/, the phonology is the same (the realization of word-final /n/ as [ŋ] is a feature of several Spanish dialects, including Andalusian and Caribbean). All sibilants are unvoiced, so that where Portuguese has /ʒ/ Galician has /ʃ/, and where Portuguese has /z/ Galician has /s/ or /θ/. And most importantly, all vowels, stressed or not, are pronounced crisply as in Spanish and unlike Portuguese, in which unstressed vowels are reduced (/o/ or /ɔ/ to /u/, /e/ or /ɛ/ to /ɨ/, /a/ or /ɐ/ to [ə]). When one hears Galician spoken, it often takes a while to realize that what is heard is not Spanish. And while in principle speakers of Portuguese and of Galician should be able to understand each other, when the great Portuguese singer Amàlia Rodrigues appeared on Galician television, she preferred to reply in Spanish to the Galician-speaking host.
The orthography of Galician, too, is based on that of Spanish: it has ll and ñ where Portuguese has lh and nh, z where Portuguese has ç (Galicians don’t fully agree on whether the name of their country should be written Galicia, Galizia or Galiza), and án or ón where Portuguese has ão.
By the same token, Macedonian and Bulgarian are fundamentally the same; indeed, until the beginning of the 20th century Slavic Macedonians thought of themselves as Bulgarians, including the famous Miladinov brothers, considered the pioneers of Macedonian literature, whose collection of Macedonian folk songs was published as “Bulgarian Folk Songs”; to some Bulgarians, to this day, Macedonian is a Bulgarian dialect. The two languages share distinctive grammatical features not found in other Slavic languages, such as the definite article and the absence of noun cases and of infinitives.
But the phonologies are quite different. Bulgarian has vowel reduction that is quite similar to that of Portuguese, while Macedonian has the five crisp vowels of Galician and Spanish, as well as Serbocroat. It has regular stress (paroxytone in disyllabic words, proparoxytone in longer ones), while in Bulgarian it is variable (including oxytone).
The threesome is completed, then, with Serbocroat, or more specifically Serbian. The Macedonian Cyrillic alphabet is based on that of Serbian, not Bulgarian (which is more like that of Russian), so that in place of Bulgarian й, ю and я (for /j/, /ju/ and /ja/, respectively) Macedonian has ј, ју and ја. It also has the Serbian ligatures љ (for /l/), њ (for /ɲ/), as well as џ (for /d͡ʒ/); and it uses — uniquely — ѕ for /d͡z/, so that it has no need for digraphs. In fact, the Macedonian alphabet is the closest that I know of to a perfectly phonetic one, being both reader-friendly and writer-friendly; and if someone wishes to learn a Slavic language with no prior exposure to any of them, I believe that Macedonian would be the easiest by far.
Except for the fact that Serbian has long and short vowels, it and Macedonian sound very much alike. I am not fluent in either one, and one time, in a taxi in Skopje, it took me a while to realize that the driver was a Serb, speaking in his language. The Macedonian and Serbian film industries use each other’s actors freely, and what is probably the world’s best-known Serbian song, Ramo Ramo (which has been performed by U2), was composed and introduced by a Macedonian.
Let me add a third case, so that I will end up with a threesome of threesomes: Danish, Norwegian and Swedish. More specifically, since once cannot really speak of a Norwegian language, I will refer to “standard” Eastern Norwegian (standard østnorsk), as spoken in and around Oslo and in most Norwegian films and television series, which is quite close to the Bokmål standard.
This language is, in fact, derived from Danish, which for many centuries was the official language of Norway and, in a norwegianized way, the spoken language of its upper crust; where it is now spoken generally, it has displaced the original Norwegian dialects (the old Oslo dialect is said to have died out in the 1970s).
The norwegianization of the standard has been gradual, and even now the Danish and Bokmål versions of the same text will appear very similar. For one thing, unlike Galician/Portuguese and Macedonian/Bulgarian, there are no drastic differences in orthography: the characters æ and ø were not replaced by their Swedish equivalents ä and ö, since Sweden, despite ruling Norway during the 19th century, did not meddle into Norway’s tangled language controversies. Nor did Swedish, as far as I know, influence Norwegian vocabulary, unlike Galician and Macedonian, which get much of their “modern” vocabulary from Spanish and Bulgarian, respectively. For ‘train,’ for example, Galician uses the Spanish tren, not the Portuguese comboio, and Macedonian uses the Serbian voz, not the Bulgarian vlak. But from watching Scandinavian crime shows one knows that Norwegian police is politie as in Danish, not polis as in Swedish.
But spoken Eastern Norwegian has very little in common with the actual phonetics of Danish, and it sounds much more like Swedish. In a film or television series in which two or all three of the languages are spoken, one can always spot the Danish-speakers, but it’s much harder to tell the Norwegians from the Swedes on the basis of sound alone. The Norwegian Liv Ullman made her career mainly in Sweden, while the Swede Stellan Skarsgård has been a star in Norwegian cinema.