RE: jr transmitter problem
As you said, you ran your battery way down by leaving it on. When you connected it to the charger, the voltage/potential was very different from that of the battery's. This caused a surge, which blew the fuse.
Fuses don't blow while your flying, radios only draw 150-200 mA while operating, not 3 amps. Nothing to worry about.
The fuse needs to be a 3amp, fast blow fuse.
Good luck.