How do you manage multiple Git profiles (GitHub, Gitlab, Bitbucket)?

Are you talking about your git name and email? Because you can set that (and other settings) per-repo simply by omitting --global in `git config`.

If you have multiple github accounts, you can use separate keys by defining custom hosts in your ssh config, but that doesn't seem relevant in your case.

you must learn to properly set up ssh via ssh config file. then use a hostname alias for each identity.

this is required online security 101 btw. otherwise you're sending your full keychain identity (all your pub keys) to every ssh server you try to connect