Parallel transistors for brightness control of large quantity of LEDs
You have the transistors as emitter followers so they are going to drop a lot of voltage. In a comment you said you have series resistors for each LED that are not shown in the schematic (necessary). Okay, they've been added but via an edit so all the comments about them being missing look odd .
I suggest using a single P-channel logic-level MOSFET. Low = ON in this case.
Somthing like an AOD417 which comes in a relatively easy to handle package.
At 3.6A with at least -4.5V Vgs it will be less than 55m\$\Omega\$ at 25°C so dissipation about 0.7W (will get warm, so a square inch or so of copper would be good). For more money, a https://www.vishay.com/docs/70297/sq40031el.pdf will be about 15x better.