How to merge PDB-files to build trimer from monomers
The PDB-file only contains the asymmetric unit, and no information on a potential biological relevant multimer. So you will need to get information about the state in solution experimentally.

That being said, one can often deduce from the size of the contact surface whether you have a potential quarternary structure. You can do this by hand, but the PISA-server is quite helpful there, and it will also output the multimer PDB-file:

http://www.ebi.ac.uk/pdbe/pisa/

If you just want to apply symmetry operators to your asymmetric unit in order to create symmetry mates, you can do so in 

Pymol:
- click Action/generate/symmetry mates/within X Å
- from the command line:
        symexp prefix, selection, cutoff
        , e.g. symexp sym,1GVF,(1GVF),5
   see also http://pymolwiki.org/index.php/Symexp

Coot:
- if you just want to look at the symmetry mates, activate View/Cell&amp;Symmetry with an appropriate radius and/or select "Symmetry by molecule"
- if you would like to actually create the symmetry mates, try Extensions/Modelling/New Molecule from Symmetry Op  - however you will have to specify the SymOp manually.


You will need to have the CRYST1-record in your PDB-file with the correct space group.