The Spread I was thinking for all three forms was:
HP: 70
Atk: 50 (-20)
Def: 85 (+15)
SpAtk: 130 (+60)
SpDef: 85 (+15)
Speed: 100 (+30)
I feel like this is the most basic spread it can probably get while still being balanced. Even though the weather abilities aren't as strong as they were in Gen 5, Castform's great coverage options and guaranteed Stab Weather Ball are enough to warrant caution. I think it makes sense for it to be more of a wall breaker so I haven't invested too much into defense.
I do really like the idea of alternate stat spread, but I think that will really affect their viability, as Fire and Ice already are at a disadvantage in terms of defensive typings. If they're all using the same somewhat Glass-cannon spread, its more of a choice based on stab and which coverage options you would need to fill out your weather team.