How can I compare three slopes amongst each other after robust regression?

Hi! I am trying to use LinearModel.fit in Matlab R2012b to robustly ('RobustOpts','on') fit a model that has one categorical variable (which defines three groups) and one continuous variable as predictors. I am specifying an 'interactions' model and would like to compare the three slope estimates afterwards. Note that I am not interested in whether the slopes are different from 0 but which slope differs significantly from which other slope. I was wondering whether there is a straightforward way of doing this, or how to do it by hand?

Also, I could not find documentation on how to switch the RobustWgtFun to 'huber'.

Any advice would be appreciated! Thanks! C.