Need to Add External Fan

[tcsmedes]

I have the PI Juice on and RPi3B+ with an Ada Fruit 3.5" PITFT on top of it, the the GPIO pins are all covered or in use. I use this unit to run an ADSB tracker so it has a SDR attached via USB, the SDR runs very hot and before I put the whole unit in a small weather proof box I want to add some cooling, or at least air flow.

So what I would like to do is run a 3.3V fan over the SDR, and I still want to power the whole thing from he PI-Juice/PI Combo. I am better off using the 3.3v pin on P3 or trying to run the fan from J3? If so what advantage do I get from using J3? Or, is there a better way to do all of this?

Thanks
-Tad