Electronic – Should I include a JTAG port on the product

The only thing relating these three phrases is that they all have something to do with serial communication.

A "COM port" is a serial port specifically in Windows or DOS. They are the only operating systems which refer to serial ports as COM.

JTAG is a serial programming / data access protocol designed for interfacing with microcontrollers and similar devices.

A UART is a chip or sub-component of a microcontroller which provides the hardware to generate an asynchronous serial stream such as RS-232 or RS-485.

The "COM Port" and "UART" are probably the most closely linked in that a UART will be used in the computer to provide the hardware that Windows sees as a "COM Port".

The JTAG is completely unrelated.

JTAG is a port like any other (you also wouldn't want to have to connect five different cables to attach a LCD). The layout itself doesn't matter although there are various standards (probably every vendor has its own standard, or two) but since it is possible to also use a high clock frequency (especially on TCK - 25MHz) you might want to route the lines together and not spread them apart with different lengths. If you want to access them via testpoints, why not place these together. Also note that it's usually possible to make a chain of multiple jtag devices. You will need to connect TDO to TDI of the next device and so on. For the JTAG port you will usually have the following pins: TCK, TMS, TDI and TDO. Sometimes you might also want to have a Reset Pin where you can reset your peripherals but this is usually no JTAG requirement. You will have to check your devices if you need a reset. Check the usual suspects for their typical connectors (ARM, Xilinx, Altera). These are pretty well specified and you can buy tools with the appropriate connectors (instead of having to make your own adapters).