Three men observe a specimen under a microscope.
The three men are scientists in a lab.
Men observing specimens under a microscope are not always scientists.  Microscopes are used in places other than a lab.
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 p1 x1 x2.(n_man(x1) & card_eq_3(x1) & in(p1,x2) & n_lab(x2) & exists x3.((x1 = x3) & n_scientist(x3)))