Went back to watch the summoning scene, and unless in Japanese there's no distinction in the phrase used between singular and plural, it seems like Rimuru meant to call 1 demon, and 3 showed up... maybe because the sacrifices were enough to bring that amount?
I don't think it's worth discussing much at this point since we have little to go on, but I kinda like the theory of the Primordial having the foresight of bringing some lackey's for sacrifices. He knew there weren't enough magiciules as it was from the get go to do the full resurrection.
Also peculiar that he showed up in the right instant in which Rimuru/Raphael was about to sacrifice himself.