Occasionally you need to re-fire a piece that already has holes drilled into it. The smaller holes tend to close up or reduce in diameter when they are larger. There is a method to resist this and still have a neat smooth hole.
To keep the holes open during a re-firing, cut a strip of Thinfire a little thinner than the thickness of the glass to be fired, roll it up tightly and put it into the hole to be kept open. Starting the wrapping around a pencil or pen makes the start easy and the roll can be tightened by holding the centre and pulling the end.
Put the roll into the hole and allow it to expand to fill the hole. It does not have to be solid. If the roll is as high or higher than the surrounding glass there is a tendency to get spikes.
This works fine on 6mm and thicker glass, but I have never tried it on 3mm glass. No reason why it shouldn't work though in my estimation if you can cut and manipulate 2mm strips of fibre paper.