Your plan for your new battery is wise.
I think 3 years for a deep cycle is reasonable in these circumstances as the battery was in a 4WD vehicle (I'd expect longer life in a stationary application), it was possibly flattened beyond the recommended 50% of capacity, and it was recharged in parallel with a starter battery.
To get maximum life from a deep cycle (in a 4WD) you should cycle it regularly but not too deeply, use an independant charge system when charging from the alternator, and recharge the battery fully on a mains charger after a cycle (the alternator will never fully charge any battery).
Of course some of these things aren't always practical, so most of us trade battery life for convenience.