I found a directory with bunch of expired domain names – the directory page where those domain names are actually has PR = 3

my plan is to redirect them to my main domain

Is it good for SEO or matters little?

Well your idea is really great, only problem is Google keeps track on such directories and websites which are either closed or blocked, and if they realized you are using them for your website for SEO reasons, then it will block your website from Search Engines, and then either black mark it or highlight it as a SPAM. So be cautious of using such tricks for SEO things, which made you cause more than anything.