Get a 5 gallon bucket and fill it partly with tank water and the rest with treated fresh water. Put the fish in and a lid on it, but be mindful they will need oxygen if it's a long move of more than a few hours, so you may have to get a battery operated air pump. You may also want to put some
ammonia-removing product in there as well. I wouldn't recommend putting 3 bettas together, but if you already had them in the tank together, I guess you have the rare bettas that won't kill each other.
Drain and break down your tank. All the contents and filter media should be placed in containers filled with treated water to maintain your biological filter, so you don't have to recycle the tank when you set it up at your new destination. It should survive for a few hours as well. Test the water in the new location. If it's significantly different, then slowly
acclimate the fish by adding some of the new water in your bucket every 10 minutes for an hour - after it's been treated of course.