Ok well say you have 3 websites, You want extra PR/rank/whatever So you put A link to the other websites on those 3, Would you get extra PR?
Don't worry about PR. If the site that is linking to you is relevant and has a lot of traffic then you'll see loads of traffic too. You will also get more PR juice than having say 50 links on completely irrelevant low PR sites.
Not generally, only if they are like PR 10 Backlinks hehe. But youll want thousands to have any impression on SEO