How much do these usually sell for?
Depends how desperate the buyer/seller is, you can buy a new BFL single (50GHs) for 1.1 BTC, They are being sold here second hand anywhere between 0.6 and 1 coin a piece, Again the cheapest price is bulk buying.
OP - Most I could offer is 1.5 coins for the 3 shipped to UK, this is based of a new antminer (180GHs) for 1.9 coins, so please don't be offended if this is not near what you wanted, I would want escrow unless you feel ok to send first, Please check out my history and feedback, I don't mess people around.
If not, I wish you well for your sale.
Best
Squall1066