I need to create or buy a seamless, 3 way transfer switch.(actually 2)

Basically I'm using 3 deep cycle marine batteries, 1 at a time while the other 2 are charging. When power drops too a certain point on the first one I need it to automatically switch power to another battery while putting bat 1 on charge. What i steh best way to achieve this? Is this something I can build myself or do you think it is going to be extremely costly? I'd like to keep the budget on it under $250.

Thanks in advance...