If you're not accustomed to moving heavy weight in the gym, then the first couple days of Project Mass have probably been eye-opening. Lifting 3 reps is a totally different animal than training in the 8-12 rep range. With only 3 reps per set, you have to lift a near-maximal load.
Today, make sure you keep going hard and heavy. You're probably stronger than you think. You have an optional rest day tomorrow, so don't think you have any excuse to quit early.
If you have pull-ups in the bag, then think about adding some resistance. Add weight with a belt, hold a dumbbell between your feet, or do very slow negatives. We'll do pull-ups for more reps later in this microcycle, so for today, make sure you're doing everything you can to make them difficult.