One -- use 3rd party software to merge the three images into a single image and import that.
Or -- import each image separately into it's own pattern. Then you can merge the three patterns into a single pattern by copying one pattern (or part of it) into another. It might be best to use a 4th pattern as the merged pattern. Then, if you have problems, you still have the original from which to start over.
Remember that raw imports are almost never exactly what you want -- take the time to tweak to get just what you want from each image before you even attempt to merge them.
The software I use to create image collages and catalogs of my fractals is Photoscape. It does take a little time fiddling around with the options to get the hang of things. When you go to save your merged images just key in the pixel dimensions (resolution) you want the final product to be. So if you want 8.50x11 inches then multiply 8.50x300 and 11x300 to get 2550x3300 pixels / image resolution. Then choose the type of image file you want to save it as. If you go with .JPG you'll want to adjust the quality to 100% because .JPGs are a lossy compression scheme.