How do i get my champs to level 3?

You need 3 garens to 2 star. 9 copies of the same champ.

Three 1 star units = one 2 star unit

Three 2 star units = one 3 star unit.

In your example: You had two that were 2 star each. So, if you want to get a 3 star Garen, you need another 2 star Garen. Once you get three 2 star Garen, you combine all 3 of them into one 3 star Garen.

The basic idea is the same, whatever level of unit you're aiming for: You always need 3 copies of the same unit, with the same level (star) to combine them into one unit 1 level (star) higher!

...stars? Edit: Realized this is about TFT, never mind.