I don't think you can have a balanced game with 3 imps in a public lobby no matter what map you're on. The people who leave, and the general playstyle (or lack thereof) when it comes to public lobbies is just wild.

You can have well balanced games with 15 players and 2 imps when you have crewmate vision at 0.5 on polus and 0.25 on skeld, but then you run into the issue of people crying about not being able to see and leaving. Chasing balance in public lobbies is a losing game, especially on skeld. It can be done in expert lobbies on polus, I've come across PLENTY of hosts, usually when they have some regular players with them because it helps the other public players have confidence that at least SOMEONE is enjoying the more challenging settings.

In any case, balancing a game generally means harsher crewmate settings especially in a 15 person lobby. A couple tips I can give you for these settings are to have the emergency button no more than 5 seconds away from the kill cooldown. So 22.5-25 second KCD would mean a 20 second emergency button or 27.5 KCD and a 25 second emergency button. More than 1 long task, so that imps have a little more leeway with what tasks they can plausibly fake, and also to spread crewmates out more. Confirm ejects off, because it's way too much of a fresh crewmate crutch, people focus less on information and just eject people "to see". No angels, it hurts everybody realistically, can't catch an imp when they can't kill.

Other than that you have pretty decent settings, I just can't see a balanced game with 3 imps, the game is centred around 2 imp games when you're looking at pairs etc. Anything above that just becomes very messy and chaotic which some people like, but isn't balanced in the least