I do not really get that site 
Also I do not quite understand the confusion. In hex it is RRGGBBAA the extra that does not show in Gimp color picker is the Alpha channel. You get a better display in Inkscape. This made in Gimp with 50 transparency. 80 hex = 128 halfway between 0 and 255 ie 50 %
so you want #123456 -> #123 , that does not make sense If you want 3 numbers, it is decimal 191, 245, 139
Also I do not quite understand the confusion. In hex it is RRGGBBAA the extra that does not show in Gimp color picker is the Alpha channel. You get a better display in Inkscape. This made in Gimp with 50 transparency. 80 hex = 128 halfway between 0 and 255 ie 50 % so you want #123456 -> #123 , that does not make sense If you want 3 numbers, it is decimal 191, 245, 139