All 3 of your models have high point costs. I would suggest dropping at least one of them and buying some rank and file troops with guns. Supermutants always want to close range and melee, so the more lead/lasers/whatever you can sent down range into them before they close the better. Having only 3 model, one of which is melee only, and Cait having a melee lean as well, is playing to the Supermutant’s strengths.
The linked scenarios seems to be very unbalanced against the survivors. Even more so if you play them vs an actual human instead of the AI. (The first scenario I lost after my opponent’s first turn when he burnt half the crops just by moving once and then standing there. The second scenario is also almost impossible to win vs a human (a mutant hound in the exact centre of the table = getting to the intel is a detection by the supermutants - and Piper has to essentially spend the entire game running from one point to the other in order to get a minor win…)