Best way to improve WiFi coverage of RT-AC66U

I have this router:
http://www.asus.com/us/Networking/RTAC66U/overview/

The coverage is pretty much fine everywhere in our house, but not so much in the garage/barn.
If so, what would you recommend? Or should all 3 of the antennas be the same?

I'd rather not have another device set up as an access point / repeater if possible.

-Jon