There are a couple of different ways you can get threesomes (or more)
If you're polyamorous, you can invite multiple partners to "Hangout as a polycule."
As someone mentioned, you can be a cheerleader, and have multiple football players approach you.
If you're headed to the River Rat, the construction workers will sometimes proposition you.
While working at the River Rat, occasionally a customer will ask you to service the table.
There are others, but those are the ones that are off the top of my head.