Here is a tip I learned from a very experienced curtain maker in another workroom which we have adopted at Denton Drapes. It helps you to form triple pleats quickly and accurately.
Begin by sewing the pleat in the normal way having calculated the pleats and spaces required. (I’m using a coloured thread here simply to show you the technique)
The stitching line should go to the end of the buckram. In this case we used six inch buckram.
But then sew in a second line a “small” third in. This means one third of the distance minus five mm. You may need to trial and error this to get the perfect position.
Finish this second stitching line one cm short of both top and bottom.
Then if you pinch the pleat at the bottom of this line it is really easy to make the triple shape
All you have do is to press down to the middle and then add first one side and then the other to create the full triple pleat.
Sew it into place and the job is done.