If 2 of 3 people in a zimun want to bentch can all 3 do zimmun and the 3rd continue eating without a safek of requiring to wash again and make a bracha? If they did a zimmun what is the ideal situation for the 3rd to do if he would like to continue to eat?
Yes, there is no problem for a third to respond to the zimmun, and to continue eating–provided this was his intention at the time of responding to the zimun.
Responding to the zimun means answering the initial zimun to bench, and waiting until the first berachah has been said (and answering amen to it). After this, the 3rd person can continue to eat normally, until he decides to bench.