Does the solubility of iron (iii) oxide increase when the pH of the water it is in decreases?
Iron (III) oxide is insoluble in water. If you add an acid (e.g. $\ce{HCl}$) it might react with the oxide and form a soluble iron salt.

How much of the oxide gets dissolved depends, amongst others, on what acid is used, its concentration and amount, and the time of reaction.