If we were to make starters, we'd pretty much have to do all three at once to make sure they're similar enough to each other to be "starters" and not just three pokemon that seem like starters.
1) Pick types. The most obvious types would be Fire/Water/Grass, but any Scissors/Rock/Paper triangle could theoretically work. The only other one I can think of off the top of my head would be Rock/Fighting/Flying. The problem with flying is it either has to have a secondary type, or not able to use Roost.
2) Once that's decided on, secondary types can be chosen for the final evolution stage. These types could be anything, or even nothing. In my opinion though, if we give one of the three a secondary type that negates their starter weakness (Water against Fire, etc), both of the other starters need to be given that neutrality as well.
This means for example, if a Fire/Grass was made (negating Fire's starter Water weakness), Water would need to be given either Bug, Dragon, Fire, Flying, Poison, or Steel. I left out grass, since if creating a Fire/Grass starter you wouldn't want to also make a Water/Grass.
3) Create a purpose for each starter. Utility, Physical Sweeper, Special Sweeper, Mixed Sweeper, Tank, or some Hybrid thereof. Perhaps even specify a main tactic, similar to how we wanted a Bulk Up theme for Revenankh.
4) Stats for the starters to correspond to their purpose.
5+) Everything from this point onwards would be handled the same as if it were singular Pokemon being done, except Movepool.
X) Movepool. Only reason to do this as a "group" setting is because you don't want one to be amazingly better than another, or one to be completely useless compared to the others, etc. Balance as much as possible, while still being competitive.