In terms of 3* the best elements for challenge events are blue/red/green mono teams. Tourneys vary by what elements and modifiers are allowed each tourney.
That said, for challenges I am keeping:
2 Gatos/Namahage/Rudolph/Brienne (1costume1 regular)
1 Ulmer, Valen Karil, Jahangir, Berden, Mnesseus, Isshtak. If I can snag 1 or 2 squire Wabbits they will be added to this. That covers those mono teams.
Aside from those, for tourneys and fillers to make different teams as well as complete a mono in all colors I am keeping:
1 Tyrum, Chochin, Gill-Ra, Melia, Bane, Kailani, Gunnar, Hisan, Belith, Shrubbear, Hawkmoon and the 2 Nashgars I already maxed before I could replace them with Rudolph.
And just for sheer fun I am maxing a muggy team #DoYouEvenMuggyBro and 1 Vodnik/Bauchan just to see how those event 3s work.
4s and 5s my rule is simple, keep 2 dupes max for grade A heroes like Grimm/Pro, 1 dupe for other important ones like healers etc, and 5s 3 max regardless of who the hero is. Feed the rest.