Crikey is right....three Best Offer can be made on an item and it does not matter when they are made.

If the first two offers are auto declined the third one can be made immediately...either as an offer or as a BIN at the listed price.

All offers/purchases can be made in a period of 5 minutes if they don't depend on the seller accepting/declining or counter offering.  Then the timeframe is up to the seller and can be up to 48 hours if the seller wants to drag it out.