Sword/Katana? is strong vs. Wooden Shield, medium vs. Thick Shield and ineffective vs. Metal Shield.
Stungun is strong vs. Metal Shield, medium? vs. the Wooden Shield and ineffective vs. Thick/Hardrubber? Shield.
The 3rd unknown weapon could be something strong vs. the Thick shield while weak vs. Wooden Shield.
Kind of like rock, paper and scissor but the stungun was rather unexpected.
Trying to pick the metal shield and lose the bios betting would have backfired but I think that Kaji chose the thick shield in the first round.