a polymer (or a molecule of a polymer) consisting of three identical monomers

In chemistry a trimer is a reaction product of three identical molecules. Trimers are typically encountered as cyclic trimers. Chemical compounds that very easily form trimers are aliphatic isocyanates and cyanic acids as an early intermediate in a polymerisation process, see also oligomers.

In biochemistry, a trimer is a macromolecular complex formed by three, usually non-covalently bound, macromolecules like proteins or nucleic acids. A homo-trimer would be formed by three identical molecules. A hetero-trimer would be formed by three different molecules.

A beam that receives the end of a header in floor framing. Unit: a stack of lumber or plywood, usually of a standard size.