As others have pointed out, the TZT3 and TZTXL do not have resettable trip fuel counters. So the only way to do what you want is to use the manual fuel management function and display "manual fuel tank" in a data box. If you always fill your boat up all the way, then you can easily just subtract the displayed manual fuel tank reading from the total capacity and see how much you have burned. Of course you need to remember to "add fuel" when you fill up or it will no longer be accurate.
In my case, I don't usually fill my tanks because they are quite large and I don't want to carry around an extra 1500 lbs of fuel I don't need. Fortunately, my engine gauges do have resettable trip fuel counters so can use those. It is a feature some of us hope Furuno will eventually add to the TZT units.