<p>I have a few 3D printers and now want to start building a custom 3D printer.
I want to build a 3D printer with multiple nozzles, and I want to make the hotend thin so the nozzles can be closer together.
What is the thinnest nozzle avalible to buy?
Are there any guides or details on how I could make a custom nozzle or modify a nozzle if I can't buy a thin nozzle?</p>

The size of the nozzle usually isn't the main factor for how close you can put nozzles together.  To keep the filament drive gear system from being the limiting factor, you would need Bowden extruders. "Then, the heat sinks and fans would be your limiting factor.  Have you considered a single nozzle with three extruders?  Otherwise, you need custom angled heat sinks similar to the three heat sinks on a single nozzle, and still a way to orient the nozzles at the same Z-height.  That would be difficult if all the nozzles are on the same heater block.  It still seems that nozzle size is the least of the issues of putting nozzles close together.
If you search for smaller nozzle sizes, you will get nozzles with smaller openings, not smaller overall size.  The threads on the nozzles are a standard size.  Thus, the smaller opening size can't be put closer together than the larger opening size.  Otherwise, you have only small variations between different types of nozzles and need room to screw them in to the heater block if you put all of them into one block.  You can get a nozzle using a 6 mm hex wrench that is smaller than one using a 7 mm hex (E3D).