The cause of some really horrible, nasty sounds from Fender Stratocasters and to a lesser degree other type of amplified guitars too. Ideally it should always be a wound string (like on acoustic guitars) since wound strings, like the other five strings, behave in an orderly manner and almost never causes disagreeable sounds. However, since the mid 60's it is usually a non-wound (plain) 3rd because modern rock and blues players want a string that is easy to stretch sideways for vibrato and pitch raising effects. There is a price to pay for this though because the pitch of the open G note calls for a wound string for good behavior. Non-wound's require lower string tension to attain correct pitch which is lower tension than optimal and this causes excessive string crash, unstable pitch and a rather unpleasant rasping chain-saw effect through tube amps. Strings with lower tension are more adversely affected by magnets tugging on them too, compounding the problems of Stratocasters with fixed strong magnet poles. I use the word compounding because non-wound 3rds produce more output and therefore require magnet stagger compensation to achieve a loudness balance with the other 5 strings. Problem is most Vintage reproduction pickups have a 3rd magnet setting designed for a wound 3rd string thus the 3rd string is invariably too loud and dominates the other strings. Consequently this overly loud string amplifies string crash, Strat-itis and the rasping chain-saw effect causing disagreeable sounds. So there you have the three reasons the non-wound 3rd string is a rogue string. Mercifully guitar players often find subtle techniques such as always stretching the 3rd to raise pitch and increase tension or always using vibrato when playing the the 3rd string. Avoiding playing the 3rd string in it's natural state minimizes these unpleasant sounds, often this is a subconscious reaction that many are not aware of and habitually incorporate these techniques into their music.
Kinman went a long way towards solving these problems by developing firstly a proprietary low strength Alnico-5 magnet and secondly a special magnet stagger with a short G magnet. This has the twofold effect of balancing the loudness of a plain 3rd as well as minimizing magnetic pull which thereby minimizes the primary and secondary causes of disagreeable sound. The offset magnet stagger is the reason Kinman fixed magnet pickups are made for either Left or Right hand players as well as for wound and non-wound 3rds.
For Acoustic and Jazz guitars it is usually a wound string because the kinds of music played on these instruments does not call for extreme vibrato or pitch raising.