A rheostat is a variable resistance resistor. A rheostat does NOT change voltage, it simply introduces additional resistance into a circuit in series with the loads. So for example, if you had one light with a resistance of 40 ohms, you would draw 120/40 = 3A, and your total power draw would be V x I = 360 watts. If you set your rheostat to 20 ohms, the total resistance of the circuit would be 60 ohms, you would then draw 2 amps through the circuit.

In this hypothetical example, the voltage loss through the rheostat would be V = I x R = 2 x 20 = 40V, and the voltage loss through the light would be V = I x R = 2 x 40 = 80V. The power of the light would be V x I = 80 x 2 = 160 watts, while the power loss through the rheostat would be 40 x 2 = 80 watts, so the total power delivered through the circuit is 240 watts, of which 1/3 is wasted through the rheostat.

Due to the wasting of power by rheostats, modern dimmers use a Triac, which varies the voltage of the circuit rather than the resistance. But the OP said he had a rheostat. The case mentioned by the OP is a little more complex than the example I gave, since the OPS said he had three lights wired in parallel, so the circuit using a rheostat would require analysis of a rheostat in series with three lights, each in parallel. Harder math, I will leave it to the OPS to do the arithmetic.