I think PR3 backlink is much better. Anyway, there are few other things that matters too, that is, backlinks from relevant sites worth much more than backlinks from irrelevant sites.
I think depend on your intention. If you want to sell link on your page, then better 1 backlink with PR3. If you want to drive traffic from google search engine, then PR is useless because PR is not impact to SERP result.
Pr3 you can get from sidebars or may be by chance if any post get good rankings but if you are able to get pr1 back link form related posts to your niche than it will work more better .
The pr3 back link means that it is a high quality link. If you have 3 from pr1, the pr1 sites probally aren't of high quality so I think one pr3 back link is better
It dependas a bit on what you want. In terms of page rank one PR3 link has about the same value than 30 PR1 links. If I had to choose between the single PR3 or about 30 PR1 I'd choose the 30 PR1 for ranking improvements in links with different anchor text. But if I want to improve the PR of my site, I'd go for the one PR3 link.
1 or 2 backlinks.. even from PR5 site can't do much to your rankings... Don't look at PR1,PR2,etc... try to get as many incoming backlinks from Relevant sites.. and hope for the best. No one knows how google ranks.. I saw sites having thousands of backlinks with PR2 and PR3
Depend on the age of the site, how many outbound links on that page, is the homepage of the site hold high PR? I guess that a relevant backlink from PR1 is better than an unrelevant PR3 site.