[Freesurfer] Freesurfer three groups: group A vs C power.

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Hi I have a question regarding how freesurfer calculates the degrees of freedom when you have three groups in the FSGD/contrast files but are running analyses on group A vs C.

For example I have an analysis with 3 groups and 2 covariates. My primary question is: is there a difference between any groups. Which I have the contrast: 1 -1 0 0 0 0 0 0 0 1 0 -1 0 0 0 0 0 0

When I write a contrast for A vs C: (1 0 -1 0 0 0 0 0 0) Does freesurfer consider this model as 9 regressors? Or 6 regressors; which would be present in a typical group 1 vs group 2 analysis that could be modeled by (1 -1 0 0 0 0) if group B didn't exist?