First off, have the pressure checked where the water enters the house. Find out from the water company what pressure it is that you should have as a MINIMUM and a MAXIMUM for residential household use. That should help the plumbers. In our house, the water comes inside in a 3/4" line, then gets stepped down to 1/2" for our faucets. (The How Water Heaters has a 3/4" line directly into it but his is the only 'appliance' that has that.) This is why you want the test done where the water enters the house. I believe my home is 40 lbs, give or take.
I'm not an expert at this but this is where I'd start. I want to commend you on getting 3 plumbers in to check this out. Stick w/that philosophy and everything will turn out just fine. My best to ya and hope this helps.