Can anyone tell me if I got 30 x PR5 backlinks to my site that is already PR3 what would the estimated PR of the site be? The links would be relevant to the niche.
As fas as I know you will get 30 backlinks from PR5. This is not a guarantee your site will be PR5 on the next update.
It is very difficult to tell. Even if you go look at some of the estimates of the number of links needed to achieve a certain PR, you have to take into account the number of links on the page. If you get the only links from 30 PR5 pages, it would be extremely good for you, whereas to get link no. 400 on a page would be only slighty more helpful than putting a link in your signature here at digitalpoint.
I read something suggesting random linking as well. The ratio 40 low PR sites to one high ranking site was used, however no guidelines for what was considered a high PR site. Also randomly linking to inner pages as well, I think the ratio was about 50/50 for smaller sites. Also random bookmarks as well.
Nobody can predict that but you will most probably get PR4. What I can say for sure is that 30 x PR5 links from quality sites in your niche will improve nicely your SERP
I agree with everyone that says only the Big G will know, you'll just have to wait and see. Give us an update down the line and let us know.