Well you would think if this build is available for the ac88u, asus would also have one available for the ac3100. Which I'm calling "the little brother" currently as over the last two months, there has been a couple beta/alpha build's posted for the ac88u, and nothing for the ac3100. I know there are no promises, but as a customer, it does bother me a bit seeing these newer build's for the ac88u, and even the ac5300. Yet nothing for the ac3100, just left out in the dark.
If Asus main focus is the ac88u, then they should think about selling it retail at places like bestbuy, instead of selling the little brother ac3100. I personally picked up the ac3100 at bestbuy, which doesn't sell the ac88u currently. Anyways enough of me going off-topic, and ranting a bit. I just think asus could do a better job also supplying build's for the ac3100. As the AC3100, AC88U, and AC5300 is there newer routers out currently. I know there's also a good chance these beta/alpha build's will cause more problems then they fix, and might make them not use able. It's just a matter of having more build's to choose from.
When developing, you try to focus on very specific models, as otherwise you end up trying to debug things all over the place, making it more difficult and more time-consuming. The fewer you work on, the easier it is. That's also how I work personally.
Also note that these aren't made officially available by Asus either, they are reposted by users. Have you actually tried to contact Asus to obtain an alpha build for the RT-AC3100 (which is the official method of obtaining it)?