We do have hadiths that state we can say the third Shahada, but scholars disagree on whether the chain of the hadith is authentic or not. Those who say the chain is strong say you can say it. Sayed Sistani doesn’t seem to consider the chain strong, so he says as a precaution one should not say it.
We the Shia should not make an issue out of this. Each person can follow their Marja’ about this fatwa and maintain unity. We all believe in the third shahada. It’s just a technical matter whether we can say it in Namaz or not.