Three boys in swimming trunks walk upstream.
Some boys are in swimwear are walking in the stream.
The boys heading upstream could be walking next to the water, not necessarily in the stream.  There is no way to be sure the body of water is a stream since the term upstream can describe moving opposite to the flow of water in any body of water with a current such as a river, brook, stream, tributary, etc.
exists e1 x1 x2 x3 x4.(Theme(e1,x4) & Actor(e1,x1) & v_walk(e1) & n_upstream(x4) & in(x1,x2) & n_trunk(x2) & of(x2,x3) & n_swimming(x3) & n_boy(x1) & card_eq_3(x1))
exists e1 p1 x1 x2 x3.(n_stream(x3) & in(e1,x3) & Actor(e1,x1) & v_walk(e1) & exists x4.(in(x2,x4) & n_swimwear(x4)) & (x1 = x2) & n_boy(x1))