Contradiction with Luke 3:16
This verse has John the Baptist acknowledging that Jesus is mightier, which doesn't contradict directly but shifts the focus from the precedence of Jesus in John 1:30.
Luke 3:16: John answered, saying unto [them] all, I indeed baptize you with water; but one mightier than I cometh, the latchet of whose shoes I am not worthy to unloose: he shall baptize you with the Holy Ghost and with fire:
Contradiction with John 3:30
This verse implies the necessity of Jesus increasing while John decreases, not a direct contradiction but a different concept than John 1:30 which emphasizes Jesus's preexistence.
John 3:30: He must increase, but I [must] decrease.
Contradiction with Matthew 3:11
John the Baptist, speaking about someone coming after him who is mightier, somewhat contrasts with John 1:30 that emphasizes Jesus existing before him.
Matthew 3:11: I indeed baptize you with water unto repentance: but he that cometh after me is mightier than I, whose shoes I am not worthy to bear: he shall baptize you with the Holy Ghost, and [with] fire: