in all honesty, this is not always a blessing for the reasons posted above. If this happens a lot, then what you truly have are three calves not obtaining enough milk from three momma cows. This cow's calf is having to share...and the momma's of the other two calves are apparently not producing enough milk for their respective calves. It WOULD be a benefit if this cow produces too much milk for her calf, and you had an orphan calf. Personally, I would separate her from the other two calves if this is a regular occurance.
By the way, nice calves! might be even better if they stick to their 4 personal udders....LOL