It is most likely a refueling formation. The vessel in the middle is a resupply ship. The picture is deceiving because of the distance but U.S. Naval Vessels are actually 50-75 feet apart. They do this regularly for getting more fuel (both for the vessel itself as well as diesel/jet fuel as might be necessary) and sometimes fresh water and supplies.
It really is a sight to see in person but from the last time I was directly involved (1994) it was still a classified process as the U.S. has the only "Navy" that is capable of refueling underway.