Multiple LTE Cards with an Linux Router OS?!

I have an simple problem. I wil rent some Train or Tran and make Sightseeing Trips. Many People around the world want Wifi. :unamused:
So that mean I have to make my own solution for. The idea is put 3 ordinary LTE Cards into an Mini Pc (and Idea which Mini Pc/ Mainboard have 3 or more PCI-E or M.2 Slots for that purpose) install an Virtual Router Software for Linux to and together with Squid the People can surf in the Internet and I have an Internal Page for the Sightseeing stuff.
The Proxy Cache the Traffic and block Bad Sites.

Now the question does anyone here try before to use 3 LTE Cards at the same Time with an Router Software?
The idea behind is to run an VPN Tunnel throw all 3 LTE Cards and send the Traffic to 2 VPN Connection at the same Time so if one connection lose there connection and the other link is ok the data arrive and Datacenter on the other side. The other Problem is it must fit into an Airline Trolley and thanks to the SSD Price I can use it instead of HDD.

So the main problem it must be cheap becouse we dont have any money right now. :blush: