Keyless Entry fob quit working?

Could you tell me the method you used to program the new remotes into the system? I normally use a scan tool to program them as I know that works. There are a few methods out there that only work on specific versions of the vehicles software.
That said the RKE receiver is under the upper dash trim panel in the middle of the vehicle. However I would use a scan tool first as it can test the module to see if it is operational and you would need the scan tool to get the replacement module to operate as it needs to be programmed into the PCM. You could also drop into any automotive locksmith and have them test your fob to see if it is or isn't operating. Did you try the resync mode? For that stand next to the car and hold the lock and unlock buttons down for 7-10 seconds the horn should beep if successful.

I would have a scan tool try before condemning the receiver. As I said there are times that the vehicle may have slightly different software or options and things don't work. Much easier to verify it that way than to take the dash apart, source a module and have it installed to discover it doesn't make a difference. Where did you get the new fobs and are they OEM or? Asking because you cannot program used ones in on most GM vehicles and with the aftermarket all bets are off in terms of parts working these days. I've had brand new parts like alternators and ignition coils that were bad when they came out of the box!

OK, so the fobs started working but then all three failed? That should mean that they are good at least. You changed nothing? IE didn't carry a different cell phone or start charging one and were not near anything like a radio tower? Was there any temperature changes between the times they did/didn't work? Heater on AC on? Asking because if something like that changed the system it would point somewhat to a communication issue or possibly cracked solder joints that are temperature sensitive in the receiver.
Changing it out requires that the two side trim panels for the windshield be removed, then the trim around the instrument panel, then you GENTLY pry up the defrost grid. Then remove all of the screws that hold the upper pad in place. Flip it up and under it you will find the RKE receiver clipped in place. I might try unplugging the connector and checking for corrosion and maybe reseating the connector a few times if the connection looks dirty. If it looks OK, then replace the receiver. Then you need to program in the remotes again.

Good to hear that it's working. Yes, the fobs all need to be programmed in the same session. That is because entering programming erases the stored fobs. It's actually a good thing, say you are out somewhere and lose the fob you have, you can enter programming mode and delete that fob so it becomes unusable.