he couldn't take oro soul coz he was dieing by the sword and he was in pain... that's the why he couldn't finish the jutsuThe answer probably might be this:
We saw that Hiruzen could not grab all of Orochimaru's soul and instead took a part of it (corresponding to his hands) and sealed it with himself in the god of death's stomach. Then Orochimaru was left without the use of his hands, but still very much alive. And later on he got the use of his hands back as well after he took on a new body.
So, it might be possible that Sandaime similarly split his soul into the three clones and cleverly tricked the god to allow three sealings in lieu of one soul. He might have known about this loophole since he, after all, was the professor and knew all there was to know of the Konoha ninjutsus.
If not this then I have no better explanation.
Out for now.
what your are saying is the best answer to consider but this jutsu is not this way... is you can use it this way it will be like gedo-mazo dragon jutsu or something... but it's a jutsu that you can seal just one soul and the cost is your life and you have only one life