Буквосочетание из трёх букв, то есть три буквы, идущие подряд в каком-либо слове. Возможно ли такое, что где-то оно передаёт всего лишь один звук? Другими словами, один определённый звук обозначается тремя буквами.
Если такое бывает, то нельзя ли привести два-три примера таких слов?
Если же нет, то как, допустим, это доказать, убедить упрямого оппонента в том, что такое явление невозможно?
Well, my initial intention was to close this question exactly for the reasons mentioned by @Quassnoi - it looks like you are confusing following concepts - "звук" (a sound) and "фонема" (a phoneme). In phonology phoneme is a unit of sound that distinguishes one word from another in a particular language. There's some argument about this, however usually in order treat something as a phoneme you need to have a pair of words differing in this phoneme (one has it, one does not, like "дом" / "том").
I'm not aware of such three letter phonemes in Russian. Russian alphabet is quite phonetic, apart from palatalized consonants, all most frequent phonemes are expressed with a single letter.
However!
We, just for fun, can examine following beasts: palatalized geminates, that is, softened doubled consonants. We do have some geminates in Russian, compare "коса" and "касса". Finding palatalized one (especially spelled with three letters, otherwise one can use dialectal "дрожжи" as an example) is a bit tricky, grepping a dictionary will help you to find some extremely exotic words like каттьерит (a metal), гюиссье (a historic term for special kind of guard), шоконнь (a dance) however with "л" you'd be lucky enough to find a a couple of more frequently used words, like "балльность", "тролль".
It's not a phoneme in Russian by any stretch of imagination, moreover, unlike the geminated "с" geminated l is quite exotic for a Russian native speaker. I can imagine and I can pronounce "тролль" myself with a long soft consonant however it would be really untypical.
But palatalized geminates is the closest I can think of.
UPD: today I woke up and realized that there's even a ль/лль pair in Russian - "коли" and "колли" )
Возможно ли такое, что где-то оно передаёт всего лишь один звук?
I'm not sure what counts as one sound.
You could say that the consonant cluster in the word килограмм-метр is a long nasal consonant sound. But in Russian, as well as most other languages, long nasals consonants are usually analyzed as two phonemes. Тонна would be [tonnə], not *[ton:ə].
In words with long vowel clusters, like длинношеее or зоообъединение, there's either [j] or hiatus between the vowels. Also, in all of them vowel reduction kicks in.
In abbreviations like ОООП, there are no reduced vowels, still there is stress and secondary stress. And it's still three phonemes.
In Russian, three letters could never denote one phoneme, if that's what your question is about.
If you insist on asking about the sounds, rather than Russian phonemes, then one argue that onomatopoeias (звукоподражания) such as «вжжик», «мяу», «тпрру», and so on, each stand for a single sound that we all know and recognize, rather than for a sequence of phonemes that only arise when we try to spell them.
