Yes, 3 fish is fine... well according to the rule of thumb 10g per fish it is pushing a bit, but it is ok... we do have 5 in about 43-45g... so much the same and they are sooooo happy.
Don't worry about the pH. It is fine.
Your filter is very good for the tank size and yes, depending how you set it... I recommend at least to turn twice the volume, maybe a bit more. So after the 50% waterchange you can wait for about 3 hours and test again and then test the following day too.
Do you have beneficial bacteria to add?
I use "Stability" from Seachem. It speeds up the cycle. We are cycling a new tank too for a schoal of guppies at this stage