How can I solve a 3 variable problem with the TLBO (teaching learning based optimization) algorithm?

Hello, I´m new with all this of the algorithms and how it works, so I can´t make to solve a problem of multilevel inverter with TLBO, can someone teach me an example of three unknown variables. In the moment I have a problem in this line:

[THD,ansr]=min(fx);

BestFx{(iter)}=THD;

BestX(iter,:)=pos(ansr,:);

and the error is this:

Unable to perform assignment because the indices on the left side are not

compatible with the size of the right side.

Error in TLBOB1 (line 82)

BestX(iter,:)=pos(optind,:);

Thank you so much.