There is no tri-band in 802.11ac

This is correct, though there are tri-radio APs/routers out there that have multiple 5GHz radios. If you really wanted to get in to semantics, you can call the UNII-1/2/2a/3 divisions of 5GHz "bands"...

Some of these tri-radio units have the ability to use a dedicated radio for mesh backhaul. Arris makes a unit like this, but sadly the software on it is a hot mess (you need an android or iPhone app to configure, you have to create an Arris account, you can't change the internal IP address range, and the list goes on...). It's sad because these could be an awesome solution for home mesh.