cheeze1,
Three batteries get you 50% more amp hours than two batteries. Hook them up in parallel(positive to positive to positive and negative to negative to negative). Charger charges all three. The batteries need to be exactly alike and all the same age (condition) to maximize effiency in charging and discharge.
I bought and installed an inverter (600 watts) to run the tv and we also carry a dish antenna and satellite box from home when we travel for more than a short stay. Then, we can watch a little CNN out in the boonies. Or whatever. Still, you will drain those batteries pretty rapidly. So, we try to just use the batteries mostly for the waterpump and lights. My inverter isn't big enough to run the microwave, so we just kick on the generator for a short run when we need to use it.
PSW
PSW