I have never heard it used in everyday speech, except as part of a quote, as in "Weave a circle round him thrice". As Gary said though, most people would understand you if you use it. Cone to think of it, I will start using it. It has such a mysterious sound.

Good question! In the US, it is common enough that most people know it but almost never use it. It sounds very old-fashioned, like from Shakespeare, and most people only use it as a joke. I would say "three times".

It's not common, but not really uncommon either. I think most people would know what it meant, or at least work it out. It sounds particularly good as 'once, twice, thrice'.

Anyone who has studied Shakespeare, or even seen a Shakespeare play, will be familiar with this word.

Oh gosh! I remeber someone once told me that this word existed, and still I had to check te meaning before recalling it XD It is not commonly used, is it?