Just like you, I also chose bitcoin and ethereum as my favorite coins. For the third coin maybe I'll choose a doge, because he's very popular lately. Maybe the prospects will be good in the future.
Dogecoin will only become popular when there is influence from outside or from third parties, because the unlimited supply of Dogecoin makes its popularity not very significant, so the third choice for Dogecoin at this point is not wrong, but for the future I personally am still doubtful.