I'm looking for some general thematic ideas for 3 decks, for my sons and I to play co-op

Ideally would be 2 relatively straight-forward decks. The 3rd could be more complicated, ideally with a support focus for the other 2

What's your collection like?

Heroes with global boosts could make fora nice thematic three-party attack. Leadership Boromir is traditionally found in a mono-Leadership deck, but there's a lot of Gondor cards outside Leadership. Lothiriel could back another, she's a lot of fun.

Leadership Dain boosts all dwarves everywhere; dwarves come in all four colors and you can mix a swarm or two with a mining deck.

Celeborn boosts all Silvan allies, they and their events come in all four colors.

Elfhelm and mounts for everyone can be fun, if not much of a theme.

With three decks you can have all nine members of the fellowship as your heroes.

or Thorin’s company of dwarves. Depending on the size of your collection you could split the cards between themes like Gondor, Rohan, Dwarves, Hobbits etc. Or you could go mono-sphere to minimise conflicts over cards. The possibilities are endless, and we know too little

Ongoing support/buff like Elfthelm Hero (bonus due to horses) is complicated to keep in mind. I'd suggest playing a healing support deck. Or something with A Test of Will. You get the picture...

Depending in how much they are into lore, you can use trait decks. But as they tend to have depending game mechanics they could be allready also complicated to play. That is why I would go with pure decks. You skip the whole resource managing part and they can focus on "classical jobs": blue for questing and red for combat. Maybe just questing might be a bit boring, though.

I like two well balanced combat-questing decks and one mono tactics deck. Tactics is relatively easy to control and is a fun to have a deck just ready to play clean up on whatever pops. One of the more successful monotactics deck I've ran in multiplayer is legolas, beregond, and hirgon. If you want to run ally light, eowyn, bard and grimbeorn are excellent against stronger enemies.