Are there any famous games where a3 was played to protect the pawn on b2 from capture by the black queen?
Position after a3.

Which opening is that? a3 can be a Queen trap but in the Sicilian the pawn can be poisoned.

You can look at the Sicilian poisoned pawn games if you want to offer/take the poisoned pawn.

I recall showing the move a3 saving the b2 pawn in an old Blog.

http://www.redhotpawn.com/blog/blogread.php?blogpostid=126

That has a might have been from Fischer - Spassky and an RHP lad falling into the trap.

Thanks greenpawn. I knew I could count on you.

Greenpawn, in the past you have asserted that a trap is not a trap if it doesn't hold consequences for the trapper if the bait is not taken. Is a3 truly a trap given that definition?

A serious loss of a tempo if not needed or best move to protect pawn

A trap for an animal can be a trap whether the animal takes it or not. So, why not a chess trap?