Is there any hope of entering Paradise for a person who led people in prayer while in a state of major ritual impurity (janabah), and repeated this three times? And what is the expiation for this sin?

Be gladdened by Allah's repentance and forgiveness, for the one who repents from a sin is like one who has no sin, and Allah loves those who repent and purify themselves. Do not despair of Allah's mercy, even if your sins reach the heavens, for Allah forgives all sins for those who repent, and none despairs of His mercy except the disbelievers and the misguided. Satan seeks to cast the repentant into despair and hopelessness, so beware of that and rejoice in your repentance. What is obligatory upon you is only to make up the prayers you missed, and you are not required to inform those with whom you prayed. For, if someone prays behind an imam and it is later discovered that he was in a state of ritual impurity, the worshipper is not required to repeat the prayer.