Legalities first:(assuming you already understand the licensing)

Planning trips north or south of the border? GMRS(as used in the US) isn't legal in either Mexico or Canada. While Canada technically has their own 'GMRS' it is functionally identical to FRS here in the states with similar restriction such as low power(<2w)/hand held with fixed antenna only/no repeaters and all equipment must be certified by their version of the FCC. I believe(don't quote me) US legal FRS radios can be used in Canada. There are also places in the US close to the Canadian border where certain GMRS channels can't be used.

Transmitting on amateur bands while abroad/near the border is a whole different subject that is best answered by r/amateurradio.

CB is legal throughout N. America due to trucking use, though technically you need a permit in MX(obtained from a consulate before crossing the border) and what channels you can use in MX are limited.

Gear next:

Packaging 3 radios in a single rig can get complicated. Packaging 3 top of the line radios will be really hard(they tend to be large). With that in mind I'd recommend choosing compact versions with remote mounts and controls on the mics.

The Midland MXT275 and Uniden CMX760 will easily cover your GMRS & CB needs for vehicle to vehicle comms over realistic distances without taking up too much room. There are also a few 2M/70cm amateur radios that have a similar form factor, but that's not my forte.