I should have added in my original post that after enabling IPV6 on the RT-AC3100 I had to reboot it before it would pull an IPV6 address. Since then it has worked perfectly.
3.0.0.4.380_3341 works perfectly for me on an RT-AC3100 on Comcast. 3941 and 63.2 do not ever seem to work for me. The RT-AC3200 that this replaced worked with every firmware I ever tried, up to and including 63.2.