How do I specify 3 interrupt transfers per microframe in high-speed USB?
Section 5.9 of the USB 2.0 Specification says:


  A high-speed interrupt or isochronous endpoint indicates that it requires more than 1024 bytes per microframe when bits 12..11 of the wMaxPacketSize field of the endpoint descriptor are non-zero. The lower 11 bits of wMaxPacketSize indicate the size of the data payload for each individual transaction while bits 12..11 indicate the maximum number of required transactions possible.