How does Combat Damage Work with Undergrowth Champion?
UC will deal three damage.

The key is that replacement and prevention effects change what you will perform. Necessarily, this happens before the replaced instructions are performed.

The event being replaced is


  All combat damage that’s been assigned is dealt simultaneously.


In this case, the above means the following before prevention effects:


  Simultaneously, UC deals 3 damage to the 3/3, and the 3/3 deals 3 damage to the UC.


But there's a prevention effect affecting the second half. Even if we na&iuml;vely apply apply the effect, the answer is clear


  Simultaneously, UC deals 3 damage to the 3/3, and remove a +1/+1 counter from UC.


But as per 615.5, the additional part of the prevention effect is actually delayed, so we actually get


  UC deals 3 damage to the 3/3, and then remove a +1/+1 counter from UC.


Either way, it's clear it does three damage.




  615.5. Some prevention effects also include an additional effect, which may refer to the amount of damage that was prevented. The prevention takes place at the time the original event would have happened; the rest of the effect takes place immediately afterward.