High Side MOSFET Driver
The way to handle this, where you use a floating supply for the high-side drive, is to size the supply for the average current, and make sure that C1 is much bigger than Cgs. When the driver turns on the FET, the power supply can't react quickly enough to supply the needed current. All of the charge transferred to the gate will come from C1 in an instant (likely much less than 1us that you quoted). The goal here is to make sure that the instantaneous drop in C1 will be negligible (say 1% or 5% of regulator output). That means that C1 should be something like 20x (5% drop) to 100x (1% drop) of Cgs.

Now, what is the average current? From your calculation, if I understand it correctly, average current is 3mA * D + 150nC * 100kHz. So that is only 18mA.

The only other thing to check is that the supply will function correctly with the value of C1 you have chosen. Documentation should spell this out. Also, the supply most likely has a built-in output capacitor. So in reality C1 might not need to be that big. But if the supply is connected to the PCB by wires, I would want to add C1 close to the driver regardless.