Gawaa-chan,
That's a pretty big leap. Matthias Corvinus and Vlad Tepes were two different people. Not to mention, the CV3 intro also clearly states that Vlad did terrible things (presumably when he was Vlad the Wallachian tyrant, before he became a vampire), but he wasn't satisfied and then started conjuring dark forces in the 15th century. So even if you make the leap and assume some elaborate identity-switching thing, you're still left with Dracula not assuming his dark power until the 15th century, whereas Matthias Cronquist had already assumed his power hundreds of years before.
Any way you slice it, the Lament plot is very contradictory to the intro of Castlevania III.