Not so. A rheostat burns out if you exceed the current rating. Calculate the current rating from rheostat wattage and rheostat resistance, using the power law W = I2R, so a 3W 50R rheostat will safely carry 245mA, 3W of fans. But a 3W 10R rheostat will carry 550mA, 6.6W of fans. And a 3W 100R only 175mA, a 2W fan. Though you can overclock wire-wound ones a bit. So if you want to run several fans on one rheostat, you need to pick a resistance to suit as well as a wattage. Check out Vishay's datasheets for more examples.