Three.    All three start at the same time and the same direction. Once the 3 reach 16.667% of the way around one, plane gives its fuel to another (A and B) and A goes back to refuel, whilst B and C go on, A then goes back to follow B/C once it is refuelled.

when B/C reach 25% of the way, B (the one that A gave fuel to earlier) gives enough fuel to C to completely fill its tank, now B starts flying back, and A meets it, giving it enough fuel to reach the island again, when this happens A and B are at the island and C is 50% around the world with a Half-Full supply of fuel.

A/B then go the opposite way to the initial direction in order to meet C at 75% through its journey (now on an empty supply) and give it enough fuel to get back to the island, with both A and B having enough fuel to get back also.

Actually just two. one to fly around the world, one to refuel at 1/4 and refuel at 3/4 of the way of the first
#2 refuels on the ground after flying back.