3.5 will work just fine. That's what I have now, and the washing machine works great.
I would steer clear of the one with the by-pass system. I got one of them and regret it. It will not stop running at times, so I had to screw the adjustment for the by-pass all of the way in. Now it works like the old one, but with more pressure.
Happy Glamping, Norman & Elna. 2008 Winnebago Adventurer 38J, W24, dozens of small thirsty ponies. Retired after 40 years wrenching on trucks! 2010 Ford Ranger toad with bicycles or KLR 650 in the back. Easy to spot an RVer, they always walk around with a screwdriver or wrench in one hand!