False Witness

What is happening to Jesus while Peter is denying Him?
Having been handed off by Annas to Caiaphas and his cronies (see Servant Girl), He is dealing with their false witnesses and false accusations: Matthew 26:59-68  59 Now the chief priests, the elders, and all the council sought false testimony against Jesus to put Him to death, 60 but found none. Even though many false witnesses came forward, they found none. But at last two false witnesses came forward 61 and said, “This fellow has been saying, ‘I am able to destroy the temple of God and to build it in three days.’ ” 62 And the high priest arose and said to Him, “Do You answer nothing? What is it these men testify against You?” 63 But Jesus kept silent. And the high priest answered and said to Him, “I put You under oath by the living God: Tell us if You are the Christ, the Son of God!” 64 Jesus said to him, “It is as you said. Nevertheless, I say to you, hereafter you will see the Son of Man sitting at the right hand of the Power, and coming on the clouds of heaven.” 65 Then the high priest tore his clothes, saying, “He has spoken blasphemy! What further need do we have of witnesses? Look, now you have heard His blasphemy! 66 What do you think?” They answered and said, “He is deserving of death.” 67 Then they spat in His face and beat Him; and others struck Him with the palms of their hands, 68 saying, “Prophesy to us, Christ! Who is the one who struck You?”

Had Jesus really said, "I am able to destroy the temple of God and to build it in three days" (Matthew 26:61)?
No, He had said, "Destroy this temple, and in three days I will raise it up." (John 2:19)

So what three falsehoods are in their accusation?
Jesus had said, "Destroy" (John 2:19), meaning 'when you destroy,' not “I am able to destroy" (Matthew 26:61), although it would be a true statement if He had made it. He also had said, "this temple" (John 2:19), meaning His own body, not the “temple of God” (Matthew 26:61) in Jerusalem. And He had said that He will "raise it up" (John 2:19), meaning His resurrection, not "build it" (Matthew 26:61).

Since they had already decided to kill Jesus and now have Him in their custody, why are they bothering with testimonies and witnesses?
"The chief priests, the elders, and all the council" (Matthew 26:59) - i.e., the Sanhedrin - is holding a court trial to officially convict and sentence Jesus.