Dearly beloved, I wish that you may prosper in all things and be in good health, just as your soul prospers.
"just as your soul prospers." It acts prosperously, namely in the state that is according to the Gospel.
Beloved, I pray that in all things, etc. He says, I frequent prayers to the Lord, that you may complete well what you do well. And just as now your soul, that is, the internal intention of your mind prospers, that is, advances in abundant works of alms, and the liberality of a bountiful mind, and the resources of money which you share with the needy, so may you always be able to lead a life full of virtues with the Lord’s help.
The Apostle writes this epistle with the purpose of bestowing praise upon certain brethren. He writes to Gaius, of whom he testifies that he receives many strangers, and for this bestows upon him great praise. For "he who does good is of God," he says (3 John 1:11). He further encourages him by the fact that he disapproves of Diotrephes, who does not wish to act in this way. He praises Demetrius, who acts in the same manner as Gaius, and gives a true testimony of his virtue. "As your soul prospers"; and it prospers in the life according to the Gospel.
[AD 449] Hilary of Arles on 3 John 1:2