FSL: 2x3 repeated measures glm design with age as a covariate

I am new to neuroimaging analysis. I am trying to find the best way to model 2x3 repeated measures design and add age as a covariate in fsl. I have looked at the fslwiki glm page https://fsl.fmrib.ox.ac.uk/fsl/fslwiki/GLM#Single-Group.2C_Three_Measurements_.28.22Tripled_T-Test.22.29, but still not sure how to model 2x3 design.
Any help will be much appreciated!