What is the ruling on repeating the Dhuhr prayer three times due to obsessive doubts about reciting Al-Fatihah, and what are the consequences of interrupting the intention of prayer while continuing to follow the Imam, and is it obligatory to inform the Imam that one did not complete the prayer with him?

The cure for obsessive doubts (waswas) is to ignore them. Doubt after completing an act of worship has no effect. Repeating the prayer due to obsessive doubts is a mistake and a surrender to them, and it involves committing the prohibited act of repeating the same prayer twice in one day. The errors you made while repeating the prayer with your colleague were a result of these obsessive doubts. The default assumption for a Muslim's prayer is that it is valid, and certainty is not abandoned due to doubt. Interrupting the prayer without a valid reason is forbidden by religious law, and you should have completed your recitation and followed the Imam instead of interrupting the prayer. The Imam's prayer is valid and is not invalidated by your cutting off your intention, because the intention of leading the prayer is not a condition for the validity of the prayer. You are not required to inform your colleague because his prayer is valid, and you must cure yourself of obsessive doubts by ignoring them.