Run two 4K and one non-4K external display from late 2013 Macbook Pro
You are outside "supported" territory with using three screens on that hardware.


https://support.apple.com/en-us/HT204154 - thunderbolt FAQ
https://support.apple.com/en-us/HT206587 4k and 5k and beyond 


That being said, you can try connecting things and Apple hardware is known to run more displays than the "support" document allows. Usually it's due to driver updates that can still juggle all the connections but that the performance isn't there or you end up with glitches like frozen screens or just not the frame rates Apple likes to see on supported hardware combination.