i'm planning to buy RTL expansion. my "group" consists me and my wife.

i know i can handle 3 heroes maximun. my wife can handle 4, but it's a bit difficult for her.

is it possible to play with 3 heroes, or the campaign will be too difficult?

Copper phase will probably be real rough, and if the OL has any intention of going for a plot/raze win, they'll likely succeed. I'd be willing to bet the final battle would be fun with just three heroes though. Probably be fine with some house rules - I'd suggest leaving chest rewards at four dice, and starting off with a Tamalir upgrade on the wall probably. But that's just off the top of my head, it'd be tough to predict all the balance issues.

It's conceivable that the Avatar battle will be much more balanced with three heroes, actually.

I would say that assuming the OL goes a little easy on copper level then both silver and gold would be very interesting with only three heroes.