Vasco,
I assume that 1) you're writing your code; and 2) you're quantitatively coding the three groups with -1, 0, and 1.
To properly handle three groups, you need two variables (plus an intercept). The following example demonstrates the coding strategy (with group3 as the base/reference level):
G1 G2
group1 1 0
group2 0 1
group3 -1 -1
With the above coding, the intercept is the average of the three groups, G1 shows the difference between group1 and the group average, and G2 indicates the difference between group2 and the group average.
Gang