How does this compare to Bloodborne and DS 3?
It's hard to say because, while the design philosophy is very similar to Bloodborne and Dark Souls, it's not a lot like those games. It's punishing, it has the same healing and checkpoint system, it has the same world design, but beyond that it's not too similar.
Rather than having multiple classes you just get Shinobi, rather than levelling up stats you get skill points to spend on passive abilities or new moves (though you can find items that can be exchanged for health or strength or defense buffs). Plus the story is a bit more straightforward than Dark Souls/Bloodborne since you play a singular character rather than a create-a-player. I would say that the combat is the best out of the above discussed games, it's also got the highest difficulty setting (with options to make it harder), and a lot more mobility in regards to jumping and the grappling hook.
I personally like Bloodborne more and I'd say Sekiro is on the same level as DS1 or DS3.
My Console Library:
PS4, PS3, PS2, PS1, WiiU, Wii, GCN, N64 SNES, XBO, 360
3DS, DS, GBA, Vita, PSP, Android
Top 6 this generation:
Bloodborne, Sekiro: Shadows Die Twice, God of War, The Legend of Zelda: Breath of the Wild, Dark Souls III, Red Dead Redemption II, Rock Band 4