yea its kinda difficult to tell what would be the most profitable ventures out there, at this early stage of the game. certainly mining will be the most reliable.
Missions themselves dont seem like they would be amazing at money making, more of a relations increasing mechanic and a way to get the mission ammo.
With combat you can obviously farm away.. but then it comes down to what will you do with the loot to maximise your profits? Recycle, maybe sell stuff? That would mean upping some other skills.
So maybe a combat mech for farming, a miner, and the third a hauler / recycler / marketeer. Possibly having the miner have some basic production skills?
But remember that any character using a station factory or facility needs to have his standings up, so you'll probably need each character doing some missions.
I suspect the best way would be to focus purly on combat, with a supporting hauling / recycling / selling character, or focusing purly on mining! Miner, Harvester, producer/hauler