Alternating Strabismus Surgery. What would provide the best alignment with least risk? Adjusting 3 muscles in one eye?

I have alternating esotropia. Left eye turns in. He said I have vertical misalignment as well. My doctor originally suggested operation on one muscle per eye (controlling horizontal movement) so that if we decide to correct the vertical deviation, he will not have adjusted more than two muscles in the eye. I don't want to risk affecting my good eye though. My question is, how difficult is it to go back and operate on a 3rd muscle? Also, why would he not do just one horizontal and one vertical?

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