A bow is usually tillered for three fingers, so you really don't have to worry about the additional strain on a limb. Also nock point location and tune will get it where it needs to be. A lot of bow makers want to know if you shoot three under or split so they can get that tiller exactly where it needs to be.
You can shoot with two fingers. I have seen people do this in the past, especially if the ring finger does not come off the string the same as the others (some people are just made this way). Using two fingers, however, makes it a lot easier to torque the string and puts additional strain on the fingers. It also increases the chances of pinching the nock if you shoot split.
In addition, the ring finger usually moves in sympathy with other two (middle and fore)so you may have to hold it back which creates additional non-needed tension in the hand and forearm.