One simple way to do this for HP is to cap HP at level 3 (or whatever), but allow the player to keep rerolling their HP, ignoring the new roll if it is lower. The average HP per level looks like this:
I've included the non-plateauing version of the d8 HD, too. But that's just averages. If you want a more nuanced look at the HP spread, here's a graph of the chance of each possible HP score for a d6 HD.
You can see how it eventually crawls up to 18, and then kind of scrunches up in front of it.
Just use the best 3 out of all your available levels. So a MU3, F1 would roll 2d4+1d8 for HP.
And thanks, anydice.com, for keeping me company at 2am and making such nice graphs.