Hi Vika, Yes, all three forms are right. The phrasal verb forms just tell us how the pillows are plumped. If they're plumped out, then they've been plumped from a flat form to a large soft form. If they're plumped up, they are ready so that you can lay your head on them. If it makes you feel any better, I encounter* the same confusion with perfective verbs in Russian. :) * Yes, in cases like this, you need to use the verb "encounter", not "meet".

For me, I would say "I'm fluffing the pillows", but I have heard people say "plump up" the cushions or pillow. I've never heard anyone say "plump out" the pillows before. I think "plump up" and "fluff" mean the same thing. I hope this answered your question.