Microwave more than one container at a time
There's no general answer. As you say, it's probably less than triple the time for one, and unless it's really small, it's probably substantially more than the time for one. But depending on the size and shape, it could be all over the place in that range.

You pretty much just have to try it and check periodically, starting perhaps after 10-15 minutes to get an idea. If it's relatively large compared to your microwave, you may also need to rearrange them periodically to get even heating, maybe every several minutes, in which case you'll get a very good idea how fast it's heating and don't have to worry about all the rest of this.