Hendrik, as a loyal FL and virus user, allow me to be of assistance:
1. Link your virus software to channel 1
2. in the virus software, make sure that in the "patch utility" tab you have selected "3 outs/no input"
3. In the virus software, in the "common" tab, on the first out of 16 channels, select under main out USB 1 L+R
4. In the virus software, in the "common" tab, on the second out of 16 channels, select under main out USB 2 L+R
5. In the virus software, in the "common" tab, on the third out of 16 channels, select under main out USB 3 L+R
Naturally you may use the outs as many times as you like on any of the 16 channels.
6. In your FL wrapper settings, assign the outs from steps 3-5 to mixer channels. Those are the USB outs. I attach a picture for reference. In my case, usb 1 is linked to channel 1, usb 2 to ch2 and usb3 to ch3. You may also try the automap feature.
Now if you are also using analogue outs, in theory at least (never tried if it works flawlessly) you can get up to 12 mono channels in your FL mixer, by selecting all the possible mono combinations instead of steps 3-5 (i.e. out 1R, out 1L....usb 3L, usb 3R)
...or 6 stereo channels by selecting all 6 stero combinations (i.e. like steps 3-5 above and out1LR, out 2LR, out 3LR)
Hope this helps!