me three

In a situation when one person says something, and another one agrees with them, he/she might say "Me too". If you also agree with them, but don't want to be seen as boring (by saying "Me too" just like that second person) or stupid (by saying "Me too too") you might want to say "Me three".

The lonesome person who says "Me Too!" after someone already says it and instead says "Me Three!" which doesn't really make any sense.

It originated from shooting three-pointers in basketball and wishing for it to go in or chanted as a celebration after scoring a three-pointer. Very similar to just shouting "three" or "get in". It can also be used in contexts where desirable things are in sight and the achievement of aforementioned things must be celebrated.
Similar hype words include lfg, we take those.
Hand gestures indicating you have just shot a three-pointer must be accompanied by saying this phrase.

Made popular by the Daily Show with Jon Stewart. When you have a one on one conversation with someone where you blatantly criticize their decisions or ethics.