I remember as a child (in the nineties) reading a story where the narrator was a little boy and how he was scared only twice a day – when the clock read 3:33. The 3's would be powerful enough (there were three of them at 3:33) to leave the clock in the boy's room for one minute and do what they want. They terrified the boy for that one minute they were free.
I don't think it was R.L. Stine, but I cannot remember the author.
Update: It definitely wasn't R.L Stine, but the story was in a similar style.
I know it was only a 15-20 page story, so maybe it was in a short story collection?