Some units, at least, were augmented to three cars before application of the orange 'V'. In fact, unit 1115 (one of the Kernow issues) appears to be one of those. The book Southern Region DEMUs (Kevin Robertson and Hugh Abbinnett, 2012, Ian Allan Publishing) shows two pictures of that unit dated February and April 1960 as a three car unit with no 'V' on the front of the DMBS.
EDIT: Before anyone jumps up and down and complains that Kernow got it wrong, the units did, on occasions, temporarily lose centre cars for various reasons (such as works attention). I believe I read somewhere that Kernow had a photo of 1115 with the orange 'V' running as a two-car unit. Certainly a few others were temporarily run as 2H when they were supposed to be 3H, even back in the 1960s.