Straight from the rulebook on page 9 under the Raider's Outpost explanation:
A 3-4-5 are docked at the Raiders’ Outpost. You use the Plasma Cannon to send the 5 back to the Maintenance Bay, leaving a 3 and 4 on the Raiders’ Outpost. To dock a higher-value sequence at the Raiders’ Outpost you must use a run of three ships that totals more than the sum of the ships currently docked there. The remaining ships total 7 so you may not dock a 1-2-3 since they only total 6. However, you may dock a 2-3-4 (total 9), a 3-4-5 (total 12) or a 4-5-6 (total 15) to use the Raiders’ Outpost.