But the look of it, is not the setup that I will use. I will for my X axis, only have one rail pr. side and not two.
Yes it is, just rotate the diagram 90deg. in your case the distance between the rails will be greater but the load distribution will be the same. for the x axis you need to allow for all the weight of the y and z axis so a greater capacity in the vertical is better, ie the first diagram. As you want to mount them on their side then diagram 2 will apply so you will have reduced load capacity and therfore require more pillow blocks to get the same effect.

So if the initial design required 2 blocks per rail then to get the same strength when mounted on the side you will need 3 blocks per rail.