Not sure what terminal you'll be flying into, but I can talk about T3 at CUN... Also, not sure if you've been at CUN before. And, not sure if you would have to collect your luggage & take it w/ you to your other terminal (I assume so ??). And, not sure if you have to re-check in at your 2nd terminal ??

Sometimes when you enter the Immigration hall at T3, the multiple lines are all backed up almost to the escalators where you come down. That's what happened July 30th when we arrived. They had Immigration officers at almost all of the booths, so the wait wasn't too bad. I think it took under 30 minutes to progress to a booth.

By the time you get through lines like that your luggage will have been pulled off of the conveyor & will be lined up on the floor near the belt it came off of. Even if you get through Immigration quickly, the luggage always comes out pretty quickly for us, so that shouldn't take much extra time.

Then, you'll walk through the Customs area. Nowadays you just walk through at T3 (no more luggage scanner) (no more red/green light to press) (& they rarely stop anyone for a luggage search, that we've seen).

Then, you go outside & get yourself to your next terminal. We've never done that so I don't know those details, but I know there are inter-airport buses that you catch (that are free, I believe).

Anyway, personally I think that 3 hours sounds like enough time for you to do what you need to. Hopefully someone else that's switched terminals will reply here !!