One question is about what you are using to measure status and how you are doing it. Another is how you have your batteries wired (with 3, I assume parallel).
If the batteries are in parallel, they should all track each other's voltage. If your status measure is based on voltage and there are differences, it indicates that the wiring and connections may need inspection, repair, or improvement.
It would be expected that the voltage would drop a little when a load is applied and then 'bounce back' when the load is removed. How much the drop and how quick the bounce back tells you about the condition of the battery bank.
The difference in ages of battery should not be an issue. As batteries age, they just loose capacity and are a bit slower. Old ones don't drag on the younger ones, they just let the younger ones work harder and don't contribute their full share. (that has implications which is why you ideally want matched batteries in a bank but that is for another discussion).
The first thing to do is to make sure you have good charging and maintenance equipment. This is the first place to upgrade.
Second is to evaluate your wiring and connections. The wire must be properly sized for the loads. Wires between batteries need to be designed to help balance currents. Connections must be properly made, tight, and free of corrosion.
If your batteries are in parallel, try each one on its own (by disconnecting the other two). Charge it up properly for 12 hours and then see how it handles the water pump and some other loads of around 100 watts. (you should be able to pull 500 watt hours from a single battery without having its voltage a half hour after the exercise be below 12.0v).