1. Balance 3 x 3
2. If they are the same, balance the remaining two and you will have the heavy one
3. If they are not the same, balance two of the heavier three against each other.
4. If they are not the same you will know which is the heavy one.
5. If they are the same, the remaining one is obviously the heavy one.
Not my strong suit but here goes:
You could do this using the displacement method (carrying the balls submerged) as long as the balls are the same size and have a radius of at least 7.35506 cm.
Water is 4 degrees C where 1 L = 1 Kg
All three balls are the same size
All three balls are spheres (not football/rugby shaped)
To save 5 kg you have to displace 5 liters of water. Therefore each ball must displace 1.6666 liters. Following the formula to determine radius of a sphere from the volume "r = cuberoot(3V / 4π)" you end up with 7.35506 cm.