BlueTooth HID circusit board design and prototype with 3 button for mobile device control

A prototype for a transmitter board is to be developed with 3 buttons for sending signals to be received via a SmartPhone app (Andoid/iOS) using BlueTooth HID profiles. Transmitter is operated with a rechargeable battery (AAA). The transmitter housing should be dimensioned as small and flat as possible. Housing design is not part of the order. The buttons should be about 3cm apart in the triangle.
Ideally, the transmission signals are protected against manipulation. A simple protection is sufficient.