Aspirin and ibuprofen are both NSAIDs with not too dissimilar pathways, so combining them won't be as effective as combining either with paracetamol. They'll both do their jobs, but if the ibuprofen is already doing its job, aspirin won't have much work left to do.
Meanwhile, taking ibuprofen can negate the effect aspirin has when taken against cardiovascular diseases. Not a huge problem if you're just taking it as a painkiller, but not everyone can combine the two. That's another potential reason to take paracetamol over some NSAIDs, though you should obviously consult a doctor if you're taking prescription medicine of any kind.