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False plotting witnesses are liable only if the judgment has been pronounced, but the victim has not been executed, since the law is "do to them as they plotted to do to their brother", and not as they actually did to him. Logic would dictate that if the defendant has been executed, all the more so the false witnesses should be executed, now that they have caused an actual loss of life. However, there is an overriding rule that one cannot derive capital punishments by logic - only those that are explicitly mentioned in the Torah are to be followed.
What does the phrase "by the word of two witnesses or three witnesses shall the one who is to die be put to death" teach? That the law of three witnesses is the same as the law of two witnesses: to be established as false plotting witnesses, all three have to be established thus.