i tried all three for my personal use. so i can’t say much about collaboration aspects. anyway:

• gitlab is old, stable and solid. and it eats half of your servers resources.
• gitea and forgeji are for me equal in experience, which is makes sense, as one being a fork of the other. i haven’t had any issues with anything, from setting up, using openid as auth method, to ssh passthrough.
• eventually i settled now on forgejo, simply for ideology reasons. it’s heavily developed, so things might change. but they have this nice attitude to not release a latest tag for docker. so you can’t accidentally update to a breaking change. had this issue with some other programs, which i spun up to quickly.
• looking forward to federation feature on forgejo!