The third would-be killer -- an 18-year-old Iraqi citizen -- was nabbed by cops Thursday night in connection with but reportedly not directly linked to the alleged plan to detonate explosives at one of the three now-canceled Swift shows at Ernst Happel Stadium in the Austrian capital.

Two other males -- ages 17 and 19 -- have also been arrested -- the older one allegedly boasted to police that his goal was to "kill as many people as possible," not only using bombs, but with knives too.

Thankfully, the cops stepped in when they did -- because this could have been a massacre, with scores of Swifties murdered. Vienna authorities say the three suspects were inspired by ISIS and two of them pledged allegiance to the terror org.

Meanwhile, Taylor is continuing her tour. Her next stop is London's Wembley Stadium, where she has 5 scheduled performances beginning August 15 to finish off the European leg of her Eras Tour.