Just like with your m17 R3, the Killer cards usually utilised the grey antenna wire although there are a few Intel cards that have a third connector. You just have to search around for one. Wifi cards are usually very cheap. $10-20. 
Hello,  It has been a while since I've posted, but the other day I was inside my 2014 17R1 upgrading ram, and I looked at the wifi card.. Intel ac 7260.. nothing fancy of course, but I noticed a 3rd antenna cable tucked next to the white cable, a grey one, which reminded me of the triple antenna setup on the killer card of my m17r3... What card would have been used to utilize that third antenna? I cannot seem to find any definite answer. I have a newer standard dual antenna card from that brand Netley on Amazon I could put in, but I was curious to see if anyone had any ideas first.. just want to use the 3rd antenna if I can. Thanks all!
17 R1, 3 antenna WiFi card?