Grabit only lets me use 3 threads.

Grabit requires you to set the amount of threads manually. The normal settings allow up to 3 threads.
In order to change the amount of threads right click your Download 2 Day connection in Grabit, and click on server properties. Then use the slider below maximum allowed connections to select the amount of threads you want.

 Every Download 2 Day subscription will allow you to use a maximum of 12 threads.