We can write the equation for the solubility of the salt AB3 in water as
AB3(s) ⇌ A3+ + 3B-
Since 0.0690 mol AB3 dissolves in 1.00 L of water, the concentration of the ions in moles per liter is calculated as
0.0690 mol / 1.00 L water = 0.0690 M
We now substitute this value to the solubility product Ksp expression
Ksp = [A3+] [B-]^3
= (0.0690 M) (3 * 0.0690 M)^3
= 0.000612
Therefore, the Ksp of the salt is 6.12 x 10^-4.