You can have more than 3 materials. if you for 1 change the sampler (You need to search this) you can have a 128 textrue samples. And if you follow the folowing, you can make as mutch as you want.
I just follow this tutorial for textrues that is the best way. I saw that the landscape demo als uses this.
https://wiki.unrealengine/World_...In_Depth_Guide
This part can be conuseing to short if a bit down. First export the spat maps als hight output and then select PNG. Make the material show in this tutorial. then look at this picture.
https://wiki.unrealengine/File:L...deComplete.png
You will se that and then you right click on one of those layers/elements, Then you will see a option for inporting inport that PNG splatmap. there you go.
And a tip you can use more than 16 texture’s tou need to search it. you you can make a massive material. and you use that as landscape material.