I interpret it to mean one of two things:
1. That you do 3 sets @ 75% of the maximum reps you did on Tuesday. For example, if your Tuesday sets went 12 reps, 16 reps, 12 reps, then Wednesday you would do 3 sets @ 75% of 16 reps, which would be 3 sets of 12.
2. You do 75% of the reps from Tuesday's corresponding set on Wednesday. So working with the same example, if your Tuesday sets went, 12 reps, 16 reps, 12 reps, the Wednesday your sets would go 9 reps, 12 reps, 9 reps
Unless Greg says otherwise, give both a try. Option 1 would result in doing more reps, so if you can manage the extra work without it affecting how you feel going into Friday's workout (no pull ups again until Saturday), then do it. If you can't swing that, then go Option 2. Remember the whole point of a mass gain workout is to stimulate muscle growth, not destroy completely muscle fibers, induce DOMS, and make you hate your arms for the next week.