for print, at A3 around 150dpi or less is enough already, maybe 150-120dpi

300dpi is overkill for A3 normally. Output blur? hmm..besides from strecthing the picture, i'm not sure what other problem would cause that

yeah, but when you go to printgate (those printing shop), they'll still reduce it down to 150, buit of course, the higher the better, no wrong in that, but normally 150 is alright

yes, so before opening a new file, make sure its set to ppc or ppi, you don't want to end up opening a new ppc file and curse why its so lag end up found out its ppc (pixel per cm)...means 1 pixel per 1 cm...if you have 10cm..10px already....1m...100px...imagine...

so on screen's work, usually we work on ppi