Test the heck out of your Trezor (with the multiple wallet types, I think?) and backups before migrating.
Send BTC and wait for it to confirm. Then BCH with the built in Armory signer.
For BTG, you'll need to import your private keys from Armory into another wallet in order to send it to the Trezor. Important to only do this after your BTC/BCH is safely migrated: Exposing your private keys is the dangerous part.
There is a lot of dubious bitcoin gold software out there too, so be safe and avoid taking the bait. The official Bitcoin Gold wallet was compromised at one point too!
Goatpig did post that he was working on a BTG signer, but I'm not sure if he was joking and I haven't read anything further since
Anything anyone else wants to add?