A 120 ohm resistor a 60 ohm resistor and a 40 ohm resistor are connected in parallel to a 120 volt power source What is the effective resistance of the three resistors?

The effective resistance of those three resistors in parallel is 20 ohms.

And it makes no difference what the power source is, or whether they're even

connected to a power source at all. As soon as those three resistors are in parallel,

their effective resistance is 20 ohms immediately, even if they're still in the drawer.