How to build a trimeric protein structure from monomeric PDB file?
There are no MTRIX cards for that molecule. This seems like the easiest path:

http://www.rcsb.org/pdb/explore.do?structureId=4G3Y

Download Files -> Biological Assembly 1

i.e.:

http://www.rcsb.org/pdb/files/4G3Y.pdb1.gz

And read that file into Coot (should you wish)