There is two different ways of using a BEC.
If plugging into a receiver that's used on a nitro it would just be installed between the battery and receiver. (The battery plugs into BEC then BEC plugs into where the battery did). If using on an electric RC then you usually would need another spot...
If you look at the description under "application guide" it tells you that a BEC is needed if installing in the TRX4 models. It's because the stock (XL5hv) ESC is only rated at 3amps.
Why do you think you need to get a 3 channel receiver? Adding a 3rd channel isn't needed if you want to use a...