As a huge Nightmare fan now, I totally understand your sentiment, and I would agree with your choices…IF the title was “big three in neo-vkei” or post-2000s bands. This is a heartbreaking thing for me to say too pls don’t flame me 
Definitely, to the younger crowd, they would already be in the veteran/older category. Still, very rationally speaking, for me, the “old” three would suggest more of the bands that came before them.
I am also aware of the fact that Nightmare came to be as a very direct result of Luna Sea’s influence, which makes me think that Luna Sea may be a more suitable band as a “big old three” band.
I’m gonna have to somewhat disagree with Diru being in the top three if I were to put X Japan and Luna Sea. Maybe a somewhat hot/lukewarm take, but Diru didn’t necessarily have that same kind of reach, but so did many bands, if I were to compare to X and Luna.
However, a third band I might put on the same pedestal is L’arc-en-Ciel, given their reach and influence.
I must say, anime is an incredible and vital conduit for introducing people to the jrock/vkei genre. Shoutout to SID, LM.C, Shazna, and Penicillin for also having anime openings and endings.