The advantages for manga reading are that many of them use furigana over the kanji so it's a way to build new vocabulary and get kanji readings at the same time. However, what manga reading doesn't do is give you much in the way of non-dialogue sentences. This means it's usefulness for grammar is questionable at times. It's NOT true that all manga is in slang or horribly conjugated Japanese although there is a certain amount of that depending on the genre you pick up. However, manga as a learning resource may be a level too easy for someone taking N3. (As a general relaxed reading thing though, ganbatte).
I still read and re-read Harukanaru Toki no Naka De (fixation with Heian Jidai) and am amazed how many little turns of old Japanese speech I've learned from it, but am still a good way from understanding Classical Japanese enough to read the Heike Monogatari in it;s original form
A lot of famous Japanese literature was written in old Japanese, too. Unless you're keen on that side of the language (which I am, though I haven't had a chance to pursue it yet), you're possibly better off setting your sites on more modern publications. Light novels and such like are a starting point, perhaps?
It's better if you can read things like magazines/bunko/etc even if just in small doses (short stories might be an option.) That way the grammar constructions you're learning for the exam will become more natural to you when reading them and will hopefully stick in your head...?
