Thinking of purchasing a Raspberry Pi 3 Model to fit a custom case. Need to know if the Raspberry Pi 3 Model B and Raspberry Pi 3 Model B+ have the same dimensions.
It depends on what you mean by "same dimensions".
The PCB dimensions, location of mounting holes and location of existing connectors have not changed.
As usual since components have been changed and rearranged, the board with components will not be exactly the same size and shape as before. This shouldn't be a problem for the vast majority of cases but there may be a few out there that place their mountings way too close to the board.
Furthermore an extra connector has been added to support the new POE hat. This may foul existing add-on boards or cases that assume only low profile components will be found in this area of the board.