What type of comparison are you looking to do? The group 1 vs group 2+3 compares the lower level to higher levels, while group 1+2 vs group 3 does the same, lower to higher. If you're not interested in doing those types of comparisons then run the nominal. To me its a question of what's your question and interest. And if you're doing nominal do you want a comparison of group 1 to group 2? Or is that not of interest? The results should be different, you're answering different questions in each one. My 2 cents
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