Well I can't show you lol. It is logical to assume he has one. His S/T technique is an MS ability. That's a technique unique to his sharingan only. The genjutsu he put on Yagura is so powerful, it was compared to Kotoamatsukami by Ao and Mei. We all know that's not a 3-tomoe ability. Plus, the fact that Yagura is a perfect jinchuriki also supports this theory.
We know that an MS is required for controlling a tailed beast, Kurama. During his fight with Hashirama, Madara controlled Kurama with what clearly was a 3-tomoe sharingan but we all know Madara had an EMS that fight.
You might ask why he doesn't show his MS physically. Well the answer is simple. He doesn't want to go blind in that eye. For one, he doesn't have any spare MS laying around because only a select few unlocked it. If he were to ever to go blind, it would make him resort to transplanting a regular 3-tomoe into his eye and 1. I don't even think you can replace a blind eye with regular sharigan and 2. He would lose his S/T technique, arguably the most op technique in the manga, forever.
Also, you might ask why he never killed Sasuke off to get his MS. Well, again, its pretty simple. Sasuke is needed to synchronize with the gedo statue. I believe the only way to sync with the Gedo, and essentially the Juubi, is if you have hatred far beyond any ordinary human. In this case, Tobi does not need Sasuke's EMS abilities. I'm sure you can find holes in this theory except for the first paragraph. I'm currently trying to perfect it with help from iShodaime.
