How can a married couple pray and work towards being blessed by Allah with triplets, and is there a specific prayer or supplication for that?

There is no specific supplication or prayer for conception, but a servant can supplicate to his Lord with whatever he loves, as long as it does not involve aggression. Allah has encouraged supplication and promised to answer it. Supplicating for many children is not an act of aggression, as the Prophet, peace and blessings be upon him, supplicated for Anas for an abundance of wealth and children. It is better to ask Allah to grant you good and righteous offspring who will be a comfort to your eyes, as was mentioned in the supplications of the prophets and the righteous.