Look in our source tree to see what is supported.
/usr/src/sys/dev/usb/serial/u3g.c
#define U3G_DEV(v,p,i) { USB_VPI(USB_VENDOR_##v, USB_PRODUCT_##v##_##p, i) }
U3G_DEV(ABIT, AK_020, 0),
U3G_DEV(ACERP, H10, 0),
U3G_DEV(AIRPLUS, MCD650, 0),
U3G_DEV(AIRPRIME, PC5220, 0)...