CRJ200 crew experienced a gear disagree message during approach and executes a go around. The gear is successfully extended using the alternate extension handle but a Hyd 3 Lo pressure message is then annunciated. Flight lands normally and stops on the runway for maintenance to pin the gear.

Narrative

Upon selection of Landing Gear Down on schedule I noticed the nose gave an immediate green indication while the mains seemed to take along time to show green. The in transit indications changed to red at the same time we received a Gear Disagree Message. I directed the First Officer to discontinue to approach as I requested vectors back out to departure to work the issue. While completing the gear disagree checklist we were able to achieve 3 green using the alternate extension handle; but then received a Hydraulic 3 Low Pressure message. The EICAS indicated 0 quantity; and 0 PSI pressure on the system. Upon completing the QRH for the Hydraulic 3 Low Pressure we declared an emergency with Approach and requested the assistance on standby. I requested a change of runways due to the length available. I contacted Operations to allow them time to coordinate the passenger services and plan for the tow to the gate. I notified company via ACARS and rejoined the First Officer on comm 1. I took over the pilot flying duties for the landing. We briefed the approach; completed all required checklists and completed a normal landing and brought the aircraft to a stop on the runway. After stopping we coordinated with Maintenance Control to have the gear pins installed before the aircraft was towed to the gate. The complete lack of hydraulic pressure would account for the lack of a normal extension of the landing gear system. Post flight examination of the aircraft reviled a large amount of fluid in the aft equipment bay.