Depends on the values. If they the 3 resistors are the same value, the wattage is tripled either way. If they are not, you would have to calculate how much current goes through each.
For example, if you parallel a 1 Ohm resistor with 2 1 MOhm resistors, it will still have a power rating of only 10 watts becuase essentially all the current goes through the 1 Ohm resistor.