What's the smallest possible tube that will work with my bikes?
If you're going to buy a tube to carry, you'd be better off getting something that fits properly. You don't want to push the limits of your tube (e.g. burst it when you first inflate it) if you're a long walk from anything else, and the extra bulk/weight isn't much. Also fitting a tube at the side of the trail isn't easy especially in bad weather (and punctures wait for bad weather to catch you out). So I would buy one tube for both 27.5" bikes (e.g. a 2.2--2.5" width) and one for the 29",  then carry the right one rather than wrestle with a 26". You've got 3 MTBs so cost isn't an issue. 

A tube for 1.3" width in a 2.5" tyre is stretched to twice its intended size, so it will be half as thick. If you're also stretching it in the other dimension it's even worse. (All slightly approximate). 

That said, tubes are very stretchy. I've seen 24" tubes in 26" tyres. I've run slightly too thin tubes for months on end. In an emergency you can get away with pretty much anything that you can get to fit. But if you're planning, plan to do it right.