You will not find 3x3 cards for MiniPCI. I have tested three simultaneous 3x3 cards with Atom motherborad. Problem is with interferences on pigtails betewan cards, remeber that with this configuration there is 9 pigtails in the box (with low rf power it worked good). On RB800 you can put only one 3x3 card (there is only 1 MiniPCIeX slot)
How you get such results (TCP test better than UDP???)
I have tested several AR9380/90 cards i.ex: HP, Sparklan, Tp-Link. As I said before there is no real 3x3 card for MiniPCI. I can recommend the TL-WDN4800 cards. Here is a small sample:
More info about above motherboard you can find here: https://www.cyberbajt.com/en/news/6/0/mimo_3x3.html