Which blue 3* heroes must i keep for Events?

I have
1x valen maxed
1x ulmer 1/1
2x gato 1/1

Should i Keep and ascend them all?

You should have put all the questions in one post. Keep and max. these heroes per color. In general, you want to keep healers, fast hitters and heroes that buff your heroes at average or faster speed. You should only need 2 per color, though if you have the roster space, 3 would be OK.

Blue: Valen and Gato.
Yellow: Bane & Malia
Purple: Tyrum, & Balthazar if you draw him out of a daily summon or something.
Green: Belith, Brienne & Mnesseus
Red: Hawkmoon, Namahage & Nashgar