Three stream 802.11ac adapters are hard to come by - The PCE-AC68 and the new D-Link DWA-192 USB adapter are the only ones I'm aware of... Apple did some devices that support 802.11ac in the 3*3:3 config on their Mac Pro, iMac, and Macbook Pro lines, but those are integrated and not drop-in cards if I recall correctly.
I don't think anybody has released a 4*4:4 client adapter yet - so the options there are very limited to media bridges and AP's configured in Bridge mode.
(FWIW - the RT-AC3200 is considered a 3*3:3 device as an AP to a client - essentially AC1900)