How to deprogram a lost three button key fob?

If you mean you lost a fob and want to remove it from the vehicles memory so that someone who finds it cannot use it, then you would need a scan tool that can add and remove the fobs. Then you need at least 2 working fobs as well. Now you connect up the scan tool and go into the BCM controls and delete all of the fobs from memory, then follow the scan tool prompts to add in the fobs that you have back into the system. An automotive locksmith should also be able to do this as well, but you would need to specify that you want all the fobs erased first. There is a way to add fobs without a scan tool, but it doesn't erase the current ones unless you have more than 4 fobs total.