Adafruit has made an I2C interface for four rotary encoders. It comes in the form of a PCB with four footprints that accept one of the most common rotary encoder forms (Bournes PEC11 horizontal style) ...
An ATtiny804 firmware that reads three quadrature rotary encoders and exposes their positions and button states over I2C as a slave device. An interrupt output pin notifies the host of state changes ...