I have site with three sub domains. Main domain is at PR 3 and all the sub domains are at PR 2. I want to promote my sub domains equally with main domain for different related keywords. What should be my best strategy? How back link juice travels from domain to sub domain or sub domain to domain? I mean to say if I add more and more back links to sub domain, will it be good for main domain also? Or should I keep on promoting my main domain and it might pull the sub domains also?
Stick to main domain. Treat the sub domains as separate SEO projects. So how much time you have dictates where your energy should be used.
Exactly sub domains are considered as a separate individual domains. So try to consider all such domain as an individual seo project and work accordingly. Links from sub domains may be considered as a back links so better consider it.