I think that you could get away with a six foot trough with a pair, but not a trio. With monitors that will certainly benefit from a larger and taller enclosure, why not supply them with one. Kimberly rocks will take advantage of all vertical opportunities you give them and you will enjoy viewing the additional arboreal behavior as well. Females can be considerably smaller, shier, and delicate than the sturdier, more robust males. A larger enclosure with lots of hides and visual barriers will although a female to feel more secure and increase feedings without male harassment.
Stock tanks are a good start and they can easier be built upon as many are currently doing. If you are not very handy, you can easily convert many front access cages by mounting them to the top of a trough. Some would require a frame for support and some can be directly mounted.
Vision cages alone are not suitable for use in monitors and I had a bunch of extras sitting around. Here is a six foot vision cage mounted to a six foot trough for a pair of kimberlys. A simple frame, cover the excess screening, mount two floods, and this enclosure have a bit more vertical than just a trough. Even taller would be better, but you get the idea. .
Photo 1: Outside shot (don’t mind the temporary holding cage on top of the unit)
Photo 2: Inside shot
Photo 3: Female enjoying some time basking, out of sight of male
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