This is common. How old are the cats? The older they are the less they'll appreciate the puppy's active curiousity. Never leave the cats alone with the puppy, and only let them meet face to face while you are around to supervise.
It'll take the cat a while to get used to something new in their territory. You shold expect hissing and swatting for a few days (or few months, depending on the cat's temperment and the puppy's refusal to learn what a swat on the head means)
Is the puppy crated at night and when you're not home? Or at the very least, is he isolated in a puppy-proof area? If not, I recommend looking into doing so.
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