Would it be a good idea to re-direct 3 of my similar sites to my main site, how would google look at that for PR and backlinks also to improve ranking.
301s sends all link juice. The problem though, at least in my experience, is that when someone finds their link isn't going to the original page they linked to, they just take it off and don't bother switching to the new one.
You do not want to use a 301 here. Simply link to your other sites using anchor text of course, but only if they are not on the same server.
Redirect is a bad idea.. just link it .. it would pass link juice and at the same time it is good for PR also.. If you use a redirect then a visitor to your site may think it's leading to someother site and they might move away from your site..
Do a 301 redirect if you care about Google and PR, link to your other sites using anchor text if you care about visitors.
If your 3 similar sites have some backlinks and PR, then redirecting them to one domain may help a little.