defining the post-hoc test contrats

Dear All,
I have a question regarding the post-hoc tests / contrasts in CONN toolbox.

I have 3 groups (AD, MCI, HC respectively) and I conducted first F tests using the [-1 1 0;0 -1 1] contrast.
I found significant results in the F tests and now I want to test the post hoc results, but I am not sure how to define the post-hoc contrast.
Should I use the basic [1 -1] contrast to see the difference between two groups, eg AD vs HC, or should I use the [1 0 -1] contrast, where I put a 0 for the MCI group.

Thank you in advance for your help!
Kind Regards
Elif