Yes I did. But I have no other evidence of another. If I had information of two guns in two different places at the same time i.e. Snyder's Bluff and Fort Pemberton then I would believe there was a third but I have not found it. Also the same units show up at both places (Snyder's Bluff and Fort Pemberton) from March to May 1863 that had contact with the Whitworth, Mainly Moore's command, a command that is listed to have had under it at least one detachment from the Pointe coupee Artillery (identified as the unit that used it at Fort Pemberton). Wall's and Davidson's Batteries both end up in the trenches at Vicksburg with Moore's Brigade but without a third Whitworth. They are identified by the company commanders name not the unit, but these match the units identified with the Pointe Coupee Artillery at Fort Pemberton. Another small detachment from the Pointe Coupee Artillery is listed under men killed and wounded under General Hebert's command at Vicksburg (9 men) but are not listed as a unit under his command at the surrender. A wild guess would be these men were killed and wounded when the their gun burst on May 18th in the trenches with Baldwins Brigade after being moved from Snyder's Bluff with Hebert's command, this is possible because no unit is given for the gun crew only who had command of all the guns (Wofford) of the 1st Miss. Lt. Art. by Baldwin's report.
This has been lots of fun trying to figure out.