Brienne is amazing — definitely one of the best 3* in the game. But you’re unlikely to ever stack her, so you’d only need multiple for War hits potentially. For your current mix of heroes, I could see you ending up making use of a second one reasonably often in War, but I wouldn’t level more than that.
Gan Ju is excellent for War, and stacking multiple on one team is very reasonable. I’d certainly consider leveling the second one at some point.
Same for Mnesseus. Even a third one (or more) wouldn’t be crazy for events.
Friar Tuck is ok but not great. I think he’s likely to be continually pushed to the bottom of your green priorities and never end up leveled. You could potentially eat him — though I’m a proponent of holding onto one of most heroes, since roster space is cheap.
I kept mine for a long time, and I wish I’d kept them still now that there’s a chance Weekly Raid Challenges might use them sometimes.
I’d definitely keep them for now.