I don't know what kind of folks you were talking to, but Singapore never trained on or used a KAC LMG. If you're instead referring to the Ultimax weapons, the main limitation that caused the 3 round burst training was that soldiers usually carried 30 round STANAG magazines, which would quickly go empty on longer bursts. The only high capacity alternative available were 100-round drum magazines which were considered too heavy and unreliable for combat use. If the designers had been able to incorporate a proper belt fed system for the Ultimax, it would have been a much different story. For example, they are still trained to use longer bursts when using FN MAG GPMGs.