If you plan on merging 3-d layers in Photoshop CC you see it’s unavailable on the 3-d menu tab right? well, must first select the layers you were working with… for example you are working on a logo project, you have text layer and a background layer right? before you merge 3-d layers you must select those two layers at the same time by selecting the text layer then hold shift and then click on the background layer, if you made it right you will see the two layers become highlighted. then when you go to the 3-d menu the merging 3-d option become available.
Anyways for better understanding of this photoshop step just watch this good turorial on how to do precisely
that Just Click Here!