One device, three children, and a fair way to divide it. How profiles, schedules and clear expectations remove most of the daily arguments.
Divide the day, not the device
Shared devices work best on a schedule. When each child has a predictable window, the negotiation happens once rather than every afternoon.
Keep learning apps outside the budget
Exempting homework and reading apps from the entertainment budget makes the boundary feel reasonable and encourages the use you actually want.