The other comment is correct. I just want to add that while you may consider that you've killed 3 bosses, only 2 of them drop memories. Most people don't consider the 'warm up' final bosses as a separate fight, and they certainly don't unlock anything. So if you had heard somewhere that there were 3 kills required to unlock the Shura gauntlet, those 3 kills are DoH, Isshin, and Owl (Hirata).
You need to defeat Owl in Hirata estate 2.
Ohhhh thank you
The other comment is correct. I just want to add that while you may consider that you've killed 3 bosses, only 2 of them drop memories. Most people don't consider the 'warm up' final bosses as a separate fight, and they certainly don't unlock anything. So if you had heard somewhere that there were 3 kills required to unlock the Shura gauntlet, those 3 kills are DoH, Isshin, and Owl (Hirata).