Their meaning, historically, has been identical. All 3 have been used against the same Communist parties, the same supporters of Actually Existing Socialism, the same Marxists. Personally, I see the desire among some left individuals to drive a wedge between the "tankies" and the "true/good/real Marxists" as a way for these people to shut down uncomfortable conversations with the overwhelming majority of Marxists around the world.

This process splits the "Marxists" that oppose AES, or advocate for reform over revolution, or support the Nordic Model, etc from the "evil" Marxists, the ones who support revolution, AES, and oppose Western Imperialism, giving a pass to the former because the former supports the status quo, which benefits Western Imperialism. Even if the overwhelming majority of practicing Marxists fit into the latter category, the former category are elevated in the West for their utility in supporting the system.

What this creates in the minds of those who think "tankie" isn't a pejorative for Marxists is utter distortion of the real viewpoints and real stances of Marxists. The "good" aspects of Marxism get pushed onto the Western supporting "Marxists," and a strawman is built up for the evil "tankies" that ends up being a mixture of interpreting genuine Marxist analysis in a negative light with absurd contradictions that don't really exist.

I don't think I'll convince you, either, but it's important for me to respond so that onlookers can at least see both points of view on the matter.