I just now saw your question, sorry about replying so late.

Most movers charge by the pound. So, when you look at the estimate, look for the estimated weight. The lowest bid could very well have been for a lowball estimated weight.

Were the estimates all for the same services? For example, did Company A include packing (even though you plan to pack yourself) but Company B didn't? How about insurance?

Movers also may offer different types of estimates. It's been a while for me, so I'm fuzzy on the exact terms, but I recall that they will do a "guaranteed not to exceed" estimate - which is usually on the high side, so the final bill will probably be lower - or they can do an actual estimate, which has no guarantee that the final cost won't be higher. Estimate types are regulated for interstate moves, so they might not be binding for your move, but if you got different types of estimates, it could easily explain the cost difference.

Whatever company you choose, make sure to check them over with the Better Business Bureau, Angie's List, or some similar consumer complaint service, because there are some shady movers out there. Also, read the contract carefully before you sign it. Some mover contracts will waive liability for customer packed boxes (so if you pack yourself to save money, and they drop all of your stuff and break it, too bad for you) or particleboard furniture.