Hello,
conjunctions in SPM are statistically valid only if the different contrasts are independent. This seems not to be the case with your data (repeated measurements?).
One option would be to show "overlapping activations". Create three one-sample t-tests for A, B, and C, use the same threshold and export surviving clusters/voxels as a binary file. See https://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=spm;deb6b67d.1207 for more information. In this case, you can only report cluster sizes, but no p-/T-values.
Best,
Gabor
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