If someone cut you off from the number 3 lane your in 2 they got partially to 1 then slammed on their brakes between both lanes you hit them from behind and they drive off who is at fault?

It depends on what state your in, most states have a law that if someone cut you off casuing you to hit them, they are At Fault. If they drove off, your best bet is to file an uninsured motorist claim with your insurance. It lies in between since you weren't far away enough to brake without hitting them. BUT they left the scene of an accident, hit and run. If you have there license plate #, go the police and report it. You might have to have a police report for your insurance company.