I don't understand how I should build the contrast for 3 groups 1 covariate.

No specific example is provided within fsgd example and also other colleagues report the same problem in the mail list.

Please could you provide us details on this topic?

Especially, how should be the g3v1.intercept and g31v.slope?

1 -1 0 0 0 0

1 0 -1 0 0 0

for g3v1.intercept.mtx

and 

0 0 0 1 -1 0

0 0 0 1 0 -1

for g3v1.slope.mtx 

are corrected?

Waiting for you very wellcome suggestions

Thanks and kind regards

Stefano