That's interesting, Tootler. It's been a very long time since I've played in a string quartet, but I did finish Grade 8 Conservatory on viola as a kid, and played in orchestras and smaller ensembles such as quartets for many years. And I don't recall ever having to alter the pitch of a third for any reason. Or any other interval, for that matter.

The only similarity I recall to what you describe with wind instruments, is that string players use vibrato - a technique of quickly moving the left wrist back and forth while holding the fingertip firmly against the fingerboard, so that the tone "vibrates" very slightly around the true pitch (sharper/flatter). It's used to create a fuller, richer sound though, not to improve intonation.