How tough is it exactly to get into a UC as a CS major?

For the second tier UCs the average UC capped GPA is in the 4.0 to 4.15 range and I would guess that at UCI, UCSB and UCD Engineering that average goes up more for CS.

At 3.6 you’re realistically looking at UCR and UCM as far as UCs goes. UCSC average GPA is around 3.9 IIRC.