Easy - you just need the LEC's OSP charts (I'm sure there is a better term for that), go back to the previous cross-connect point and put a tone on each individual pair and trace them that way!

In all seriousness, if you are using only 3 of the 50 pairs, there is a very good chance that that 50 count is bridged to multiple locations, and any fooling with it (shorting or letting it get wet) could affect up to 50 other subscribers. Contact the phone company ASAP to get it fixed right.