Firstly, may I add that it is interesting to note how popular slide software is becoming.

We have modelled mass concrete, gabion, soldier pile walls and have got fairly good outputs from slide that we checked against hand calculations. The results were not far off depending on the mechanism of failure you were looking for.

When it comes to other wall systems like stacker block (dry block) walls we tend to stick to the manufacturers software that is provided to give us a stab at the preliminary design.

When I use SLIDE to model MSE walls it is as a check for global stability, and I only model the grid or fabric in the slope (no wall facing). The manufacture's software was used to find the grid length & strength needed for the wall's internal stability. A three foot wall might not even need grid, so I'm not sure that SLIDE can be used to model such a small gravity wall (with meaningful accuracy)... it would be more like modeling the greater area of the slope that has a wall feature in the geometery.

What I usually do is that I model the wall (concrete or MSE) with infinite strength properties.  I start with small dimensions of the wall and increase dimensions until I obtain a proper factor of safety.  This is because as stated by jmgray, the internal stability is another design.  So by this approach, I only get the needed wall dimensions in order to obtain a reasonable global stability.