I have 3 separate runs of 10 gauge (3 wires) attached to my 30 amp shore power plug receptacle on the boat. I want to install a smart plug but there is not enough room for all those wires in the new smart plug. What would be the correct way to join these? Should I use some kind of junction box and have a single set of wires connected to the SmartPlug?
Your idea of junction box is correct, make sure it is sealed and only accessible with tools to prevent it from opening easily. Terminate the wires onto a terminal strip and then bring a pigtail to the smartplug.