I was just wondering if such a term exists, since they are very similar to each other, and all of them derive from the Greek alphabet, so I thought perhaps there might be a collective term for the three of them.
Some people in typography and grammatology use terms like Euroscript and acronyms like LCG (akin CJKV for sinograms). The L may be replaced by an R for roman and the order may be different, e.g. GRC since there is no canonical convention.
Bicameral scripts would mostly cover these as well, because the distinction between uppercase and lowercase letters is rare elsewhere.
Note that some other scripts may also belong to this grouping, especially Armenian
Armn and Georgian
Geor, but also Glagolitic
Runr and probably more.
Just for the record, ISO 15924 does not contain a collective 4-letter alias code for
Grek. It does record variants like Church Slavonic
Cyrs or Fraktur
Latf and Gaelic
We can trace the development of writing systems in family trees, just as we do for languages. The families of writing systems often don't have names though, so you could just say Greek-derived writing systems, Greek itself being developed from the Phoenician Alphabet.
However as you are excluding many writing systems that also developed from Greek, what you're asking about is really a paraphyletic group. And as your criteria for inclusion or exclusion seem to be based on your personal subjective opinion of how similar they are, I think you can't expect there to be any term for what is essentially your own private and eccentric categorisation.