36 months is a good point to check for accuracy of pronunciation.
At 2 my youngest had no 'language problems' - i.e. had wide vocab, and put together lots of words into sentences, but had some sounds missing, and was therefore hard to understand. But by 2 yrs 9 months he was much much clearer, and could say all his sounds, and mostly did.
My eldest was the same, except that at 3 he still had whole rafts of sounds completely missing - s, sh, f, v, p, t, c, g, h, ch, r, l, ...! He was very bright, had huge vocab., big imagination, and no-one could understand what he was saying. (I could only understand about 50%). He finally started regular speech therapy when he was nearly 4, and made huge improvements over just a couple of months.
(Unfortunately he reached a plateau, and for the past year has not made any progress, and still can't say c, g, or r.)
Anyway, hope this helps, and yours manage to speak more clearer by themselves by the time they are 3.