Only 3 starts sounds like it's definitely time to change the batteries.

Best advice: Replace both - one weak battery can drag down a good one. Fit the largest and highest CCA ones that will fit.

Optima or other expensive AGM/sealed batteries are nice if you can afford it (I had a red top in my YJ), but not necessary unless you are doing steep climbs and they are spilling. Just buy a good name brand with a good warranty that you can get locally. I got mine (Energizer Max) at WalMart because of a good warranty and WalMarts are everywhere and open late.

From what I hear deep cycles batteries in a car aren't supposed to fully charge and require a trickle now and then. They also don't deliver the current a good starting battery will (more important in the cold.) A deep cycle is meant for long, slow drains. IMHO, a deep cycle isn't needed in an automotive starting/winching rig that already has dual batteries.