I only had to kill the one next to the tower and the one near the spiritspring to unlock the seal

The turtles seem to be bugged. Once you read the hint in front of the door, if you fast travel at any point, there's a chance only the turtle nearest to the tower will spawn. I was able to fix it by reading the hint then riding to the other 2 turtles. Takes a while, but it worked for me.

Def. bugged. I found the one next to the tower, and did not see the third one, but after a few times jumping on the spiritspring suddenly a message appeared that the seal was opened. (I did not look for the 2nd one).

flying t-pose giant ghost turtle. Elden ring has it all

I think I have a fix.

Okay, first off interact with the seal and door again . But do NOT fast travel to get to the other turtles. Go there manually. I tried this and it worked for me . Hopefully it will work for everyone else. Good luck !

Bugged, the second turtle seems to die on its own. Only needed 1. and 3.

So if anyone is having issues with this - I just got the spell and I thought it was bugged also;

I killed the one hanging off the edge of the cliff beside tower, then went to the jump pad one, then went to the last one but it wasn't there, so I jumped down to swing randomly just incase but nothing, so I had to fast travel back as you cant jump back up and seal was still shut. Thought, **** I bugged it, but instead I just went to grace, waited for night, read the imp statue again - I checked first turtle and he was not there so I really thought I ****ed it, but then I went to the last one again and lo and behold the big blue bastard was there. Sniped him with a crossbow and the rise opened.

So I guess just make sure its night time and dont stress if they arent showing up.

For me, the seal broke after I killed only the turtle west of tower. I threw a bone dart at it, and it disappeared instantly. Definitely bugged as others have already mentioned.

If needed check all turtle locations even launch yourself for the 3rd one
The quest is bugged and some turtles can just straight up be dead at the start of it, some people said that using the book at the tower again caused them to show up I don't know for certain but the quest has issues