If there is a parallel circuit with three branches, each branch with 3 equal LEDs (and a resistor with negligible current flow), and 100mA going through each LED, what is the current flowing from the source?

Current splits up in a parallel circuit inversely proportional to the resistance of each branch; i.e. a branch with very low resistance will get the most current whereas a branch with a comparatively high resistance will get much less (as electrons, the charge carriers, generally always take the easiest route).As all branches contain the same components, their resistances are equal and so the current from the source splits evenly across each branch.Current cannot be used up and so if 100mA flows through each LED, that means that that single branch of 3 LEDs was provided with 100mA from the source. As there are 3 equal branches, the total current flowing from the source is 300mA.