How does Combat work now?

I wanted to check, does combat work the same way as in Gal Civ 2?

I especially want to doublecheck how defense works. In Gal Civ 2, you got the full number against the right weapon, otherwise you got the square root.

So a 9 armor, 9 shield, 9 chaff ship fighting missiles would get 3 + 3 + 9 = 15 defense

If fighting 15 missile attack, 15 defense would negate it for the most part, with some damage getting through.

Does 3 work the same way?