I have three independent groups in which I am needing to compare their genders (categorical variable: male, female, unknown) to see if they are statistically different between the groups. What statistically test should I run? I'm using SPSS.
ANOVA is probably the best, but you also need to select a post-hoc test such as Bonferroni or Tukey. The overall ANOVA test will tell you if there are significant differences and then the post-hoc will tell you which differences are significant between groups.