If the husband gives a gift to one wife, must he give the same to the others?

Gifts beyond obligatory maintenance need not be equal according to many scholars, but making them equal is strongly encouraged to protect feelings and avoid any sense of favouritism.
Unfairness in gifts can cause jealousy, friction and a feeling of not being valued. The Prophet's ﷺ warning against a husband who inclines towards one wife (Sunan Abu Dawud 2133, sahih) is a reminder that inclination should not show itself in actions. If circumstances justify a difference, for example a gift when a wife gives birth, give it wisely and in moderation. See Fiqh of polygyny.