Generally, before I was using BH to get the first round of 4* covers, so shifted them around on who was close to that. At present though I’m concentrating on Gambit as some of my 3* are coming up on the 2nd 4* cover but that will put them at over 6 in that. Otherwise my only level 3* Champ below 183 is Elektra and I have so save iso to champ 4* version before popping her cover and she is way down on my list.
A lot of good suggestions already out there. Obviously, the choice of 3* BH becomes a little tactical once you've got the tier champed.
With the recent addition of specific 5* covers to certain 4* champ rewards, it might be worth keeping Kamala on permanent BH status. She grants Medusa covers via champ rewards, and Medusa grants Bolt covers. As far as I remember, no-one in 3* gives covers for 4* Gamora (3* still giving 4* Quill covers) or Vulture (yet), so Kamala is the only feeding chain right now.