What are you other three fingers doing? I tend to grip with thumb on the right edge, index finger resting on the back and last 3 fingers holding the right edge.
Are you an amputee?
Do you mean "last 3 fingers holding the left edge? That style seems to be a minority, based on a quick Google image search. If fingers are wrapped around a phone, the index is with the rest of the 3 fingers.
This is how I hold my phone http://thumbs.dreamstime.com/z/isolated-woman-hand-holding-p...
Check out the image search for examples. Most hold it the way I'm describing: https://www.google.com/search?q=holding+phone
> Are you an amputee?
no