You can 4 man all the instances as long as yeh know the bosses and yeh follow the instance progression.

look at http://www.wowwiki.com/Instance under advised lvl or check out http://www.wowhead.com/?zones=2#0+2+1
dont be trying to run shadow labs before yeh run all the others before it, a few times

there's no way to 3 man the intances unless yeh really geared but if yer that geared there wouldnt really be any reason to run most of the instances.