Pienso como Jazzi aunque nunca he trabajado nada con tres hebras. Lo que si hago es hacer el pespunte un tono mas oscuro que la parte donde va perfilado y queda mejor
It is entirely up to the look you want. I stitched some ornaments over 2 on 32 count, so it was 16 count. The pattern said to use 1 strand of floss, but because of the look of the ornament I tried 2 lines and it looked much better. But for some of the small detail areas I used just 1. In one place I even used 3! I would say in the 14 - 18 count range, test and see which looks better for the effect you are going for.