Thanks PBulls, that was the solution, I was able to get it work by just adding the vname(). I didn't want to use column numbers because they could change as I add and remove columns. I didn't need to add the .sum part to the variable names.

proc report nowd data=x;
column cat -- medb dummy;
define dummy / computed noprint;
compute dummy;
    array avar(3) biga smaa meda;
    array bvar(3) bigb smab medb;
    do i = 1 to dim(avar);
        if avar(i) ^= vname(bvar(i)) then call define(bvar(i),'style','style=[font_weight=bold]');
    end;
endcomp;
run;
;quit;