The left hand frame should be the same position for violin and fiddle playing. In my experience, there are a lot of fiddle players that don’t like to take time for technique and good posture. If you want to be a really good fiddle player, you will want to pay attention to good posture habits and technique. You will have better sound this way and can also avoid injury that can result from bad posture habits.
It is true, some fiddles have shaved down the bridge to a bit flatter angle. This is not necessary to play triple stops, but I can see why a fiddle player might choose to file the bridge down. Classical violinists play triple stops, and even quadruple stops all the time. The trick is a very full and confident bow stroke to bring the sound out of multiple strings.
Suzanne from Violin Tutor Pro