I have not found any clear reference to panchakaccham (PK) in the oldest veda, the rigveda. The bhAhmaNa texts which deal exhaustively with various yAgas etc., also prescribe only two pieces of cloth for the yajamAnan and his wife and do not explain further as to how the lower piece is to be worn; the upper cloth called upaveetham (upper cloth, literally) had to be worn compulsorily, during the yAgas to cover the upper torso. Possibly, this upper cloth shrank as time passed and came to three threads (probably representing the three vyAhritis bhUH, bhuvaH & suvaH) and became the "yajnOpaveeta" or the common Poonal, but there is no clear evidence for this.
Panchakaccham gives a lot more ease of movements for the wearer, compared to the dhoti, but as things stand today, the vEshTis are a bit transparent. This makes us mamas to wear an inner wear which shows our all too hideously! In the olden days tabra men wore only Koupeenam and so this ugly site was not there, but something else, lol!
Unless the tabra man wears PK and the married woman wears maDiSAr of 9 yards, all the poojas done will go waste and only sin will accumulate, is the orthodox belief. You are free to believe and comply with this or face the sin (of violating this rule) as and when it stares at your face!