During the Summit War, Marco clashed against all three of the Admirals on an equal footing. He managed to hurt two of the three, showing that he's indeed capable of fighting and possibly even beating them.
Yamato was capable of holding back Kaido and that's all an Admiral has to do. Kid and Law took down Linlin, two Admirals would decimate her. Once that happens, Three Admirals at the same time vs Kaido would destroy him in a minute.
Shanks is a powerful pirate captain, but it is unlikely that he could defeat all three Marine Admirals at the same time.
The Admirals and the Yonko are certainly not equal in power and the former will always reign supreme as the strongest in the One Piece world, which Oda once again showed in the previous chapter.
The rank of admiral (or full admiral, or four-star admiral) is the highest rank normally achievable in the U.S. Navy. It ranks above vice admiral (three-star admiral) and below fleet admiral (five-star admiral).
I don't think you can simply say "he can beat an admiral" because even within the Admirals, there's such a huge difference in power. I personally think that Luffy could probably just barely beat Aokoji now (or in the near future), and he could fight but not beat Kizaru, and has no chance of beating Akainu.
Garp is easily the most powerful Vice Admiral in the Marines, and rivals or exceeds the power of the known Admirals and Fleet Admirals.
Being admirals, the three are capable of using Haki and showed themselves to be extremely proficient when using advanced Busoshoku Haki together to block Whitebeard's powerful shock wave.
Luffy is close to Shanks in terms of power and it is only after their fight that he will officially be able to call himself stronger than Shanks.
It is a known fact that Kaido has lost to his fellow Yonko and the Navy a combined 7 times and been captured a total of 18 times. While Kaido might be the strongest in terms of raw strength, there are definitely those who can beat him a fight. Write for Us!
Garp is a Vice-Admiral of the Marines. Being their strongest known member in history, there's no denying that Garp is capable of taking on Kaido.
Though he is a Marine and is loyal to the World Government, Garp is disgusted by the World Nobles and refused the promotion to admiral due to the fact it would make him their direct subordinate.
Although he is only a vice-admiral, Garp is the strongest Marine in the series, surpassing even the likes of Sengoku and Akainu.
Monkey D. Luffy has mastered all three types of haki, including Kenbunshoku Haki (Observation Haki), Busoshoku Haki (Armament Haki), and Haoshoku Haki (Conqueror's Haki). He can even use several subforms of the hakis, which makes him one of the most powerful haki users in the One Piece world.
He is can use all three types of Haki and back in his prime, his Armament Haki was powerful enough to help him split an entire continent of ice. Chinjao lost his drill in a fight against Garp, who claimed to have reduced 8 mountains to dust while warming up for this battle.
Like his grandsons Monkey D. Luffy and Portgas D. Ace, Garp is one of the very few people in the world who can use all three types of Haki. He is also a capable teacher, as he taught both Koby and Helmeppo.
Before he can finish, he is shot to death by Pooh Percy (now an Ellen Jamesian) inside the Steering gymnasium in front of Helen and his wrestling team. An epilogue summarizes what happens in the lives of the other characters after Garp's death.
Whitebeard, on the other hand, easily equaled Roger's power. Even Garp called Whitebeard "The King of Seas." So, it is beyond any shadow of a doubt that Whitebeard could have defeated Garp in a fight to the death.
Saving Ace would have gone against Garp's loyalty to the Marines. After all, Ace was not just any pirate; he was the son of the Pirate King Gol D. Roger himself, and his execution was seen as a crucial step in the fight against piracy.
Blackbeard is the perfect counter to Luffy's Gear 5 abilities in the One Piece series. With the power of darkness from the Yami Yami no Mi, he can directly cancel out Devil Fruit powers via physical contact.
Mihawk is said to be close to the same level as Shanks and is even dubbed as the greatest swordsman, so to say Luffy would defeat Mihawk is still debatable in the eyes of many fans.