If you've already got the plugs then go for it. When I got mine I found that it cost very little extra to buy made up leads off ebay. The plugs\sockets are about £10-£15 each plus cable and 3pin plug.
You're right about joining the phases together in the plug.
If your tester doesn't perform a polarity check first then connect the live and neutral together.
This will prevent someone trying to power a 3 phase appliance via a 3 pin 240v single phase supply
You are aware that there are different types of 3 phase plugs?