Bayern Munich and Ajax can win the 3 titles in a row, too. But only Real Madrid who could win 5 titles in a row (1956, 1957, 1958, 1959, 1960). This record is unlikely to be repeated by other teams because it will be very difficult to win UCL in 5 consecutive seasons. That's why many people claim that UCL is Real Madrid league, Real Madrid is too dominating this competition.
Bayern Munich, Ajax Amsterdam, Real Madrid and Liverpool are teams that have some dominating seasons in the past but I did not implied about these seasons. It's too far in the past and European football last two or three decades are very different than in 1970s, 1980s and before that.
Since the change from C1 to Champions League, only Real Madrid successfully defend their Champions League title, and this fact shows how hard to become a Champions League winner last three decades.
Even the golden generation of Messi can't dominate UCL. They are defeated by Ronaldo era that was managed by Zidane. Barcelona may dominate La Liga in the era of Messi, but they never can dominate UCL. Real Madrid is surely in another level when it is about UCL. I think people call Real Madrid as the king of UCL is understandable. Sure, no team looks to catch the record of Real Madrid in UCL
It's true so and three consecutive Champions League titles of Real Madrid with Zidane as a head coach, is very magnificent achievement for their club and it's an impressive club performance in Champions League history too.
Manchester City failed to defend their title in the recently completed season too and this again shows very high competitiveness of Champions League. It's hard to win it but it's even much harder to defend your title.