I've seen this error recently, when I had both FieldTrip and SPM in the Matlab path (the problem was due to having multiple versions of fieldtrip functions in the path, because SPM includes parts of FieldTrip)...
Edit your Matlab path: remove everything that is not related with Matlab (remove all FieldTrip, Brainstorm and SPM folders), save and close Matlab. Restart Matlab, do not add any path to Matlab, go to the brainstorm3 folder and run brainstorm.m, then run your test.
Please tell me if it solves your problem, and what you think the conflict was.
I probably need to add additional checks somewhere...