Scoop's matching algorithm will always try to match you in a 3 person carpool by default. However, depending on the 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, Shortlist is not available to Drivers who are already matched in an existing carpool. Additionally, if you are an unmatched Driver, it is only possible to select a single rider from the Shortlist.