If there's a tester which requires 3 people to test (for example in minecraftcity) and the tester shows red, can all the people in the tester be killed?
No. You are not 100% certain whether the person is a traitor or not. However, if you know two other people are proven by some sort of means, you can apply common sense and use process of elimination.
If its the tester I'm thinking of, if the three are all traitors, it will show up as green so you obviously can't kill the three because you'll think they're proven. Tbh that tester is probably the worst tester there could be.