Personally I switched from MR816 to an RME-Audio interface long time ago. Without changing anything else on the computer I was able to work trouble-free, the constant dropouts disappeared.
I use the MR816 as a AD/DA converter now. Also the two headphone outputs come in handy from time to time.
If Adat lightpipe is ok for you you could get something like the Digiface and then connect all 3 MRs via Adat.