Is it not #"NH"_3#? If you don't see it, draw out the Lewis structures.
Perhaps you should use this website more often. It shows:
That is, a #120^@# rotation about the #z# axis returns #"NH"_3# invariant.
On the other hand, #"C"_2"H"_2# and #"CO"_2# belong to the #D_(ooh)# point group, and #"SO"_2# belongs to the #C_(2v)# point group. In english, the first two here have infinite-fold rotation axes along the #z# axis, and the third has a two-fold rotation axis on its #z# axis.
#"C"_2"H"_2#, #D_(ooh)#
#"CO"_2#, #D_(ooh)#
#"SO"_2#, #C_(2v)#
Neither of these point groups contain three-fold rotational axes.
