In your line

> sigma3(x) :=[op1+op2]*[op3+op4]*[op5+op6];

try replacing brackets with parenthesis and do expand():

> sigma3(x) :=(op1+op2)*(op3+op4)*(op5+op6);

> expand(sigma3(x));

Don't know if this was what you were looking for.. but anyway :)