Compare it to this also:
Can you use cost down to reduce the number of tributes needed to trigger the "if you tribute 3 monsters to summon this card" effects of Moisture Creature/Gilford the Lightning?
Nope. But you could summon it without tributes with 2 cost downs. Doesn't add up to 2 cost down and 1 tribute to trigger the effect. For that matter, there is a ruling about if you use Cost Down with Majestic Mech (the 2400 6 star one) that even if you use cost down, Majestic mech still dies at end phase unless you tributed for his summon. Cost down has no effect on this.
I would say the same goes here. It says very clearly the ONLY way to summon Victory Dragon is to tribute 3 dragon sub types.
I'm glad it's this way though. A monster that can end 2 games in one swift kick should be that hard to get out. I don't want to get hit with VD anytime soon. I would do whatever I could to stop it. But if you look at the SJ that it came in, there are several good ideas to swarm the field with dragon sub-types in less than a turn, if set up for properly.