afni.nimh.nih.gov

> If I remove one of the three contrasts it would show me a similar omnibus test?

Yes, it should.

> I could use gltCode to compare two groups as well like a post-hoc test, right?

That's right. You can also specify the omnibus test for the three groups using gltCode since it's a test with one degree of freedom:

-gltLabel 1 'omnibusF' -gltCode 1 'group : 1*short -1*intermediate & 1*short -1*long' \

Gang