RE: hevyist weight for 3 sets or drop weight?
If the whole point of your training is to push your body through it's current limits then lifting progressively heavier weights for full sets in the run up to your heaviest set will negatively affect your ability to lift the heaviest weights possible in your heavy set and therefore affect your ability to push your body beyond your limits.
I would recommend warming up thoroughly and then blasting out your heaviest set, then reducing the weight to further fatigue the target muscle group. Remember though that a warm-up is exactly that... a warm up! that does not mean going all out or feeling fatigued at the end of these sets and they should let you feel fresh going into the heavy sets that matter.