The water in my tub is always pristine. I add a few gallons of water every now and then as it looses some. I treat it well and the readings are always where they are supposed to be. My question is do I still need to replace the water entirely every 3 months?

It's not necessary to replace it at any particular interval as long as it doesn't build up a lot of dissolved solids, and your chemistry stays on track. My own tub, with good monitoring and limited useage, I've gone six months at a lick without problems. Just watch the ph and alkalinity levels.