CRJ 200 pilot reported hydraulic system low quantity indication in cruise. Flight crew diverted and landed safely. Hydraulic fluid leak was found post-flight.

Narrative

I was a rated captain serving as SIC (Second in Command). While scanning system synoptic pages on the EICAS; the PIC noticed a low quantity indication of about 35% on hydraulic system 3. This was northbound and approximately abeam ZZZ [Airport]; enroute from ZZZ1 to ZZZ2. Initially; we suspected the low quantity indication may simply have been due to air in the system and we elected to continue the flight while monitoring the situation. Within about 10 minutes; however; the system 3 quantity declined further to 20%. At this point; we believed a significant system 3 leak to be confirmed; the most critical of the three aircraft hydraulic systems; and elected to divert to the nearest suitable airfield.We chose ZZZ3 [airport] since it was the nearest to us; only about five minutes away; and ZZZ4 [airport] would have been more than double that at about 12 minutes. At the time of the decision to divert; hydraulic system 3 was still showing adequate pressure and we hoped to land before a complete loss of pressure in the most critical of our hydraulic systems.During the descent into ZZZ3; however; the 'HYD3 LOW PRESS' EICAS amber message illuminated and a check of the system pressure indicated about 1200 psi. We ran the QRH; turned on the 3B system; as directed by the QRH; and confirmed system 3 pressure remained low (less than 1800 psi). Since the system 3 pressure remained about 1200 psi; and the QRH directed 'land at the nearest suitable airport' we elected to continue the descent; approach and landing into ZZZ3. It had a long; 9500 ft runway; visual weather conditions; including crosswinds less than 15 knots; good services; pax handling capabilities and advanced maintenance services.We advised ATC and requested delaying vectors from ATC while we completed all the QRH procedures; including manual extension of the landing gear; and made preparations for a visual approach and landing with questionable braking and ground steering capabilities. Fortunately; we were able to land normally and use differential braking to exit the only runway at ZZZ on the high-speed taxiway; preventing other aircraft in the traffic pattern from having to also divert.Vehicles met us on the taxiway; clear of the runway; and directed us to set the parking brake and shut down engines. The parking brake was able to hold by turning on hydraulic system 2B allowing use of the outboard brakes.The ground crews noted a fluid spill around us which they believed to be fuel; but was subsequently determined to be hydraulic fluid. Eventually; we received a tow to the general aviation ramp and our passengers deplaned into the FBO for further handling by ground crews. Since this was otherwise a normal landing and pax deplanement; no further aircrew actions were necessary.