The Cisco/Linksys RV016 (just under $400) can group up to seven different WAN connections, but the customer reviews are only so-so. For a little more I can get a Peplink Balance 30 that can handle three WAN connections and seems built for speed.
There are other products, I know, but not a lot of information about any of them. And while I can see a user point out that load balancing “seems to really slow the speed significantly,” I can’t see much user discussion of Peplink. Netgear offers products that can group two WAN ports, but I want at least three for some reason. Richard Mackney, however, wants to group a bunch of USB cellular modems.