3 years seems pretty good for using a car battery outside of it's preferred use case. I guess that depends on how good/bad you were about deep cycling it.
Currently, the batteries I'm using are my power tool batteries, which are 18V so they charge through a dedicated (12V) charger, and I have a little USB A/C and low powered inverter that uses them. I probably wouldn't necessarily want to put my lithium batteries through every day cycling, though.
I've thought about generating hydrogen with it to use for experiments and such, but idk if I have the space for that.