It's always good to exchange links with sites that have better PR than you...but, make sure that those sites are relevant to your site.
i doubt there will be very many PR7 sites that will be interested in exchanging links with PR3 sites. but if you can find any, then my all means you can exchange links. link exchange doesn't help much in terms of SEO these days, but getting a backlink from PR7+ site (even from inner pages) will have some benefit
On one of my sites, I've managed to get a PR 8 site to give me around 1,000 backlinks, and it immediately went to PR 5. To answer your question, yes, you can really get lucky with it.
Yes, it's really good. However some sites with have higher PR (like PR7) don't want to exchange links with sites that have lower PR. Some of them want to provide same PR. But if your site has quality content and relevant to their site, they might be considered you as their link partner.
Yes it's good to do .. both most PR7 sites would be selling their link space rather than going for link exchange..
Is it good to do link exchange with websites that have pr7 or over if you site has pr3? Yeh Definitely It's True !!!!
Not if they want to KEEP their PR 7, they don't. Selling links on your site is the BEST way to get your Page Rank reduced.
Yes nice to have pr7 link in exchange of pr3, but it's hard to get one. You are getting like a prime minister's reference rather than from a chief minister.