Of course 3 days per week of leg work outs is possible. Some might say it is too often, but others have done it for years with good success. What is healthy is going to vary with the individual and of course volume and intensity are also relevant.
Considering the PO original message, this is not a question of whether one can work out 3 or more times per week but a question of "do you have to?"
I've known some exercise freaks who worked out every day; one guy in particular would ride mountain bike after work for couple hours, then he'd go to gym for another hour. But this was just a warm up for his daily ice hockey game. God bless his soul, but is this really the life you'd want?
Just to be a better diver, you only need some strength to demonstrate every once in a while. Like to lift tanks and gear, to climb the boat in high surf when the ladder does its best to shake you off, or maybe even carry a double load of tanks and gear plus your wife/GF sometimes.
To achieve this, all you need to do is lift weights twice a week. Do 4-5 sets of squats or deadlift (your choice) and 4-5 sets of bench press, both large weight, low repetitions (3-6 reps, best of all in "reverse pyramid" order i e 1 set 8 reps 100 pd as a warm up; 1 set 3 reps 125 pd; 1 set 4 reps 120 pd; 1 set 5 reps 115 pd; 1 set 6 reps 110 pd) etc. As for the pull-ups, best of all make them part of your daily routine. Install a bar at home and do for example 1 set before breakfast, 1 after breakfast, and 1 before dinner. This is all you need, plus your morning warm up, of course (crunches etc, maybe 10-15 min).