No... Portrait A4 folded in 3 should mean that the folded size is 70x297mm!
Do 'they' mean a 6pp A4? Which would be a flat size of 630 x 297mm that folds to A4 portrait.
Landscape A4 folds in 3 to a 1/3 A4(DL) leaflet and is usually termed 6pp 99x210mm (or 6pp DL - but that's not strictly correct!)
Hope this helps....