You need to figure out where the missing coolant is going to. Is it leaking on to the ground-could be water pump going out-usually can spot leak at seal adjacent to the belt or the water pump's sealing gasket. The coolant could be coming out the exhaust pipe, which is good indicator it is blown head gasket. Warm car up and see if there is significant amount of white smoke coming out the pipe. The coolant could also be going into the oil pan, possibly due to blown timing gasket. Check oil level with dipstick, see if level has risen.
Chris
