I have the relevant descriptive statistics for each of the three groups. I also have the stats for the pooled +1 groups and the -2 group. Any direction would be much appreciated.
I'm not sure this is what you are looking for, but in SPSS you can request a partial eta squared for the contrasts you are interested in, using GLM or UNIANOVA syntax:
UNIANOVA y BY group (if you have a covariate use GLM y BY group WITH x)
It's not clear from your description what +1 and -2 refer to? If you are estimating effect size for a continuous variable, you'll need the means/sd and sample size of each group. If you ran an ANOVA and this is for post-estimation, you'll need the df and F value.