We bought a home that already had a sewage pump installed in the basement with three drains already installed in the concrete floor that leads to this pump's tank. One was for a toilet (which is covered), another is for a sink (which at present is in use) and the third is a drain pipe (rubber cover) about 2.5 inches in diameter and sticks 3 inches out of the concrete floor. It is the last one that we want to use as the drain to a shower.

The third pipe leads directly into the sewage pump tank that is located within the basement floor. My question is, "how do you install the base and trap for the shower?"

You should repost this in plumbing for Tom to properly answer.

I added a shower in a basement with the same setup. I built the floor up around the shower to allow for the drains.