There needs to be more information about the protein.  Have you been following RG questions using the search box?  Rajul Tomar has a very similar question:   "My protein exists in monomeric,dimeric, as well as tetrameric form and tetramer is the only active form. Major part produced is the monomeric form (more than 90%). Its a recombinant protein expressed in E.coli. Is there a way to make more of the tetramer?  

Would you be able to answer the questions from the other readers of Rajul's inquiry?

The trmer is the naturally abundant form? If yes you might only need to adjust the buffer conditions (ionic strength and pH). If not it might become more difficult. 

Protein concentration will be another way to shift the equilibrium toward oligomeric form.

Thanks for your suggestion. I will try with Schubies idea and even I got one paper says about the similar studies and will update the results also

'Macromolecular crowding' by the addition of PEG or dextran will mimic a higher concentration of protein, shifting equilibrium toward the trimer.  Modest [ammonium sulfate] has been used to enhance biospecific cohesion as well, presumably by enhancing the hydrophobic effect.