I always change weights between flyes and presses. Flyes are isolation moves, focusing on the chest, whereas presses also engage the anterior shoulder and tricps, so it's typical to use more weights on presses than flyes. I don't even worry about using the 20# Cathe does on flyes: for me, it would be too heavy and I would risk injury. The highest I go on flyes is 17.5#. But on presses, I usually go to 25# in PUB.
On "flyes" at 8#, you're obviously refering to the rear delt flyes. Keep in mind that your shoulders are already somewhat pre-exhausted because they help with chest work (anterior shoulder) and back work (posterior shoulder), so they've already gotten worked out somewhat, even though you weren't focusing on them. If you go heavy in chest and back, your shoulders might be ready for a break!
You might want to give your shoulders priority by doing the shoulder work first after the warm-up. It's not something you would do all the time (because it will then make the chest and back work harder because the shoulders, which help both muscle groups, will be pre-exhausted), but it's a way of helping to increase strength in a specific muscle group.
Or you could split PUB into chest/back one day, then shoulders/arms another day (don't forget the warm-up and stretches on both days). That way, your shoulders will be able to work when they are fresh.
Here are a couple of things you can try to break past the 3# mark. You can even try them together.
1) you can increase your weight by a small increment (add 1# and 2# for a set at 4# and a set at 5#)by using wrist weights.
2) Increase your weight for the second set, do as many reps as you can, then take a short break before continuing to finish the set number of reps (10 or 8). Make sure to use good form and avoid any body English that would divert the emphasis away from the muscles you are focusing on.
Above all: focus on using good form, on using a weight that is challenging but doable for you, and don't give a second thought to the weights that Cathe uses. She might be able to lift heavier because of the way her muscle inserts into the joint, for example: something that you are born with. It doesn't matter. If you continue to work with good form and maximum effort (again with good form!), you will make progress.
