Can't you find them mathematically? If I remember correctly, they are all three spawned at 120° angles from each other. If you give me just a second, I'll break out my paper and pencil and find the theoretical coords of another.
Edit: Who said vectors weren't useful?
There should be a stronghold below (694.846, 0.4891). You should probably round both of those up and dig there.
Edit²: the second number should be negative, I think, but it literally makes a block difference.
I have an end portal but am interested in this knowledge for future reference, I had an idea there where 3 end portal but the 120° is knew - and a question, I know that with 2 points I could work out the 3 but how do you work out in which directions 2 arms go? Are you saying that they surround 0,0?