Make mask texture that is RGB in this way: R 1/3rd on the left, G 1/3 in middle, and B 1/3rd or right. So 3 stripes, each 1/3 red, green, and blue.
Now see how to use linear interpolate node in material. and interpolate your 3 texures, with that mask.
This is simplest way.
You can also create that RGB mask out of expressions directly in material. With IF statments:
- get UV coordinate node. Filter only R or G just one, to get UV in single direction.
- set result color to GREEN
- and then if UV is < 1/3 set it to RED
- if UV> 2/3 set it to BLUE.