Maybe someone else can give a better answer then me, since I've only been getting back into the loop more recently. But, Issue with Groudon + Blastoise is that sun weakens your Blastoise and Blastoise + Palkia seems redundant, don't really see much point in it. Blastoise is most common on Calyrex-S + Zacian-C compositions with good reason, very good synergy on these compositions, it gives a Dynamax option, and can check Landorus-T, Incineroar, and opposing Zacian-C. Now, I don't think it's impossible to use Blastoise on other compositions, but, it'd probably be harder to justify. You could potentially use it on lets say an Yveltal or Dialga team. Yveltal is typically good into sun which Blastoise can struggle with, and Dialga may appreciate the landorus-T answer. But, shared Electric weakness with Yveltal and contending as a Dynamax option with Dialga may make it had to justify for most. So Calyrex-S + Zacian-C is probably your safest bet, but, I think there's some room for experimentation.
EDIT: Though, whatever you do with Blastoise pair it with Zacian-C, seems too good to give up