It's not that simple, you get less amp-hours from larger loads than you do from smaller loads.The capacity of the battery isn't measured in amps, it's measured in amphours so watch your terminology =) A 50AH battery is rated for 50 amps of current draw for one hour fully charged. The trick is the 50AH rating is usually based on how much current you can draw over a 20 hour period. So 50/20 is 2.5amps for about 16 hours. You're drawing 3 amps which should be pretty close if that's what's actually being drawn, so your battery when fully charged should be lasting 15 hours or so. It will slow down towards the end of the discharge, and if this is a car battery it's probably not a good idea to discharge it all the way, you'll need deep cycle marine batteries. How long do you need to run the pump for?