Is it possible to setup a bungee server with a 1.12, 1.10, and a 1.7.10 pack?
Or at least with a 1.12 and 1.10 pack.
Yes, with HexagonMC bungee or Waterfall.
Keep in mind that a single user will only be able to connect to one of the servers at a time. You’d need a BungeeCord plugin that can move players to different servers depending on their Minecraft version.