Scoop's matching algorithm will always try to match you in a 3 person carpool by default. However, depending on your route's trip demand at your scheduled time, you may only be matched in a 2 person carpool.
Can I add another Rider from the Shortlist?
Unfortunately, the Shortlist is not available to Drivers who are already matched in an existing carpool. If you are an unmatched Driver, it is only possible to select a single rider from the Shortlist at this time.