Burners

Hey Guys,
I am using three bayou burners (10 inch cast iron) on the single tier system I built. I am using one standard propane tank. I have three of the 104quart/26 gallon heavy duty brew kettles from more beer. It is taking me 120 minutes with only one burner running to get to 212 degrees with 15 gallons of water in one tun. (water temp at start is 64 degrees) Where did I go wrong? Propane Tank size / aftermarket regulator / line length / line size "id" / all the above???
Any suggestions...

What is the BTU output supposed to be for the burner? What kind of regulator are you using (high pressure, low pressure, PSI)? Are you using orifices on the burners, are they the right size? These all might be the culprit.