Raspberry Pi 3 Mini Server Rack Sled

@jonthemiller came up with an amazing rack mount for raspberry pi devices, the Raspberry Pi 4 & 5 Mini Server Rack Case, but unfortunately did not have a raspberry pi 3 to test with. I've taken his model, amazing work, and adapted to work with the pi 3.I've included one for heatset inserts, and screws, as with @jonthemiller so pick the one that best fits your needs, and print! EDIT 4/6/24Final tweaks to the I/O ports for better alignment.Adjusted insert and screw post alignment and the s