I am not a SEO expert so what ever I say could be wrong but I don't think it matters how many sites you have pointing to the same place. Wouldn't they all be treated as separate sites?
It's not like you are creating entrance pages that point to your site.
Let me explain myself for this thinking.
Say you have domaina.ext, domainb.ext and domainc.ext
When you enter the url for domainb.ext that is what the browser will display and not domaina.ext. Same for domainc.ext. That is as long as you are not doing something to make it display domaina.ext.