Imagine you had an army of 1,000,000 soldiers
Then an army of 100,000 tries to attack you
How many soldiers do you bring to fight them? Do you bring 100,000 to hopefully just barely win? Or do you bring 1,000,000 to ensure there is a 0% chance that you lose?
Sengoku summoning 3 Admirals against Whitebeard was equivalent to crushing an enemy with completely excessive and overwhelming force. No 1 Yonko can defeat even 1 Admiral.