If there's a gauge or tension on the pattern you can work backward from that to figure out what kind of yarn you need.
According to this list, http://www.yarnfwd.com/tension.html
, 3-ply yarn gets about 8 stitches per inch. That would put it in the category of modern fingering, sock or baby yarn.
If the pattern has no gauge, divide the number of cast-on stitches by the width of the sweater front. For instance: if the sweater front will measure 18" across and you are told to cast on 144 stitches, 144 divided by 18 = 8 stitches per inch.
Go back to your LYS, have them check the math, and find a yarn that gives you the right gauge.