Compatibility of 3 inch watertight enclosure

Can we replace the four inch electronic enclosure in the blue rov with a 3 inch one. Or would the 3 inch be small for the electronics hull.

A 3" WTE will physically fit in that location, but you will need to engineer new, longer cradle extensions to hold it in place.

Electronics wise, you will be pressed for room. Nido Robotics made a custom carrier board to be able to fit everything into their 3" WTE on their Sibiu Nano.