The only thing that occurs to me off the top of my head is that you have gtk2 terminator installed, and are running/testing gtk3 terminator from a local path. The command above would search the PATH, finding the gtk2 version, which wouldn't work. You would need to give a relative path (i.e. ./terminator for the current directory) to the gtk3 terminator. That's about the only suggestion I have. Otherwise, it should just work.
The only thing that occurs to me off the top of my head is that you have gtk2 terminator installed, and are running/testing gtk3 terminator from a local path. The command above would search the PATH, finding the gtk2 version, which wouldn't work. You would need to give a relative path (i.e. ./terminator for the current directory) to the gtk3 terminator. That's about the only suggestion I have. Otherwise, it should just work.