Matthew,
By "slope" across the three conditions, do you mean the linearity (or linear trend) among them? If so, code "task" as an ordinal variable:
A -> 1
B -> 2
C -> 3
3dLME -prefix ... -jobs 12 -model "group*task+age" -qVars "age,task" -ranEff '~task' \
-qVarCenters ... \
-gltLabel ... -gltCode ... \
-dataTable \
Subj group age task InputFile \
s1 g1 20 1 s1_A+tlrc \
s1 g1 20 2 s1_B+tlrc \
s1 g1 20 3 s1_C+tlrc \
...
Gang
