It really depends on your alliance, both in terms of the sort of opponents you face, and how coordinated you all are with War.
In a mixed level alliance, newer players with less leveled rosters typically make the biggest contribution by cleaning up damaged teams, and sometimes by using mono stacks of unleveled heroes to try to blow a hole in a team, particularly the tank.
For cleanups, stacking 2-3 of the same color will typically work well, as it’s hard to do sufficient damage fast enough with a weaker rainbow team to not get killed, or have your efforts undone by a healer.
For mono unleveled stacks, it’s a definite gamble on getting a decent board, and requires some good board play skills to make the most of what you get. But even completely unleveled 1-1 3* heroes in a stack of 5 can do a remarkable amount of damage, particularly if you stack in the strong color against the hero you intend to take out for busting a tank or cleaning up a team with only one or two heroes left.
So really, team selection will vary for each War and each hit, as you tailor your team to the opponent. The biggest general recommendation I can make is not to think of your teams as fixed, with 6 rainbow teams that you always use in the same combination.
On a related note, your bench is shallow enough currently that it would behoove you to hold onto all 3* heroes, even if you don’t level them. They’ll be useful for filling in for stacking.