A CRJ-90 Captain reported low #3 hydraulic system quantity was detected on taxi out. They returned to the gate for maintenance.

Narrative

External preflight was normal; no abnormalities noted. During hydraulic panel procedure portion of cockpit flow; temps; pressures; quantities were normal; Hydraulic System #3 quantity was at 50% with pump(s) on. During taxi-out; while conducting Before Takeoff Checklist; Hydraulic System #3 Quantity was observed at 35%; well below normal. Contacted Maintenance and returned to gate per instructions; monitoring Hydraulic System #3 quantity enroute to gate. Hydraulic System #3 quantity once back at the gate now indicated 30%. Made logbook entry. After returning to the gate; a puddle of hydraulic fluid was observed on the ramp in the area underneath the hydraulic panel; which must have formed after exterior preflight; since my First Officer was back on board the aircraft prior to when I performed the hydraulic panel procedure; turning the pumps on for the first time that day. Within 10 minutes of gate return; passengers were deplaned. After extensive troubleshooting; Maintenance deferred Hydraulic Pump 3A; serviced Hydraulic System #3 quantity; also downgraded aircraft CAT Status. Received new; amended Dispatch Release; boarded remaining passengers; departed again and completed flight without further incident. My First Officer did a fantastic; professional job the whole trip. In this example; during the Before Takeoff Checklist; he took the time to really study the Hydraulic Synoptic Page for a moment; catching the below normal system three quantity.