index ratio doesn’t work that way. If you have a single object cloned 15 times in a single cloner and a attach a multishader material with 15 images inside it it will put 1 image per clone. The only way (I can think of at this time in the morning), with that setup, to get an image that covers the first 3 then the next 3 etc. is to have only 5 images in the multishader then each image is placed on 3 clones. That’s why it’s called index ratio. 15 clones and 15 images gets 1 clone/image, 15 clones and 3 images gets 5 clones/image….
Either your setup is different, in which case we’d need to see it, or your first post had an error in it and you meant to say you have 15 clones and 5 images in a multishader.