First of all, check to see if the designer mentioned any thing about "PURL 3" in the pattern. If there is an explanation to it, then follow it. If you couldn't find any explanation, then I would say you can just do regular p3. The only thing I could think of this is to separate each set apart so that knitters can see it more clearly. But, I guest sometimes it turns out to be a more confusion.
Be careful doing "yo" and "p1", make sure you wrap the yarn after bringing it to the front. Otherwise, you'll end up with just a p1 without a yo.