Three men observe a specimen under a microscope.
Scientists working in a lab
Scientists are not the only people using microscopes.  Microscopes are not always in a lab.  The men may not be working.
exists e1 x1 x2 x3.(under(e1,x3) & n_microscope(x3) & Theme(e1,x2) & Actor(e1,x1) & v_observe(e1) & n_specimen(x2) & n_man(x1) & card_eq_3(x1))
exists e1 x1 x2 x3.(a_topic(x1) & in(e1,x3) & n_lab(x3) & Actor(e1,x2) & v_work(e1) & (x1 = x2) & n_scientist(x1))