Interaction/Moderation effect in ANOVA - variable is not one of the factors

I am running an univariate ANOVA in SPSS to test my hypotheses for my 3x2 between subjects design experiment. The factors for the ANOVA are review valence (3 levels) and presence of brand (2 levels). I want to see how these factors affect purchase probability.

I also gathered data for the brand commitment of each participant in the experiment. Brand commitment is measured by giving a score on a scale from 1-9(fully disagree/fully agree) for 3 statements (taken from previous paper). Brand commitment is high/low if the scores are in the upper/lower third of the scale.

I want to understand what is the interaction between brand commitment and review valence, and how it affects the DV. I predicted that brand commitment will moderate the effect of the review on the DV.

My question is how can I add this brand commitment "interaction or moderation" in the ANOVA or should I use a different method to analyse it. Also, should I compute a new variable that combines the 3 statements in one new variable.

Any insights that you can provide me with, will be highly appreciated.