Late to the party as always. The reasoning between three fixed lift engine vs two was Vought's desire to cut cost by using two Allison J99 lift engines in place of three Rolls Royce R.B. 162 engines which was estimated to save $20 million in the prototype program.
A little more on the engines The J99s were expected to have a thrust of 8510 lbs with an allowable 5% bleed to provide roll control for the aircraft in hover and transition. The combination lift/cruise engine is the P&W JTF22A-30 a version of the F401-PW-400 that was anticipated for use in the F-14B by 1978.
Vought ultimately offered two versions of the V-517 for consideration.
V-517A was to be powered by a TF30-P-100 modified for lift cruise for program cost and risk reduction.
V-517B was to be powered by the F401-PW-400. The aircraft was essentially the same as the V-517A with the exception of an airframe mounted accessory gearbox, and the risk of delays in the engine development program. These was also the mention "the Navy may desire to develop the swivel nozzle for the F401,"
Mark pretty well drained the pool of illustrations, I have a couple scanned from a view foil presentation that are attached including an Isometric view, V-517A General Arrangement and the production version (IOC 1978) General Arrangement for your viewing pleasure.