What is the ruling concerning the three marriage contracts whose guardian was a man who denied the existence of Allah?

Denying the existence of Allah, the Exalted, is clear apostasy. And whoever affirms this after embracing Islam becomes an apostate.

What this man did while he was a Muslim is valid, such as his marriage contracts with his nieces, unless he affirms that he did not believe in the existence of the Creator, the Exalted, at the time of contracting with them. In that case, the contract is invalidated according to some scholars, such as the Shafi'is.

If the invalidity of the contract is proven, it must be renewed. However, it is first necessary to ascertain whether the man was a disbeliever at the time of the contract or not.

Beware of declaring someone a disbeliever, as it is very serious due to the rulings that result from it.