I bought Heavensward when it came out. I bought Stormblood when it came out. (Actually I pre-ordered both of them)

Going by logic: Can I get a discount on Shadowbringers now, because I already own two of the three expansions they're selling you?

I was under the impression that it is a courtesy by SE to include previous expansions into the newer once, not a real "bundle" they're selling you. You didnt pre-order FFXIV: From Ishgard to Shadowbringers, you pre-ordered just Shadowbringers and your mainly paying for that. The other two are a bonus, specially for people who come to the game later.