This invention refers to a linear single chain recombinant trispecific antibody (scTsAb). This trispecific antibody is composed of an anti-carcinoembryonic antigen (CEA), an anti-CD3, and an anti-CD28 (CEA-scTsAb). More specifically, a Fc linking peptide fuses the anti-CEA single chain fragment (scFv) to the anti-CD3 scFv, and an HSA linking peptide fuses the anti CD3 scFv to the anti-CD28 heavy variable chain (V H) in tandem. A c-myc-tag and/or (His) 6-tag is attached to the C terminal of the CEA-scTsAb either alternatively or simultaneously. This invention also refers DNA sequence coding for the said scTsAbs, plasmids containing the said sequences, and host cells containing the said plasmids. These invented scTsAbs are intended for use in the development of therapeutic drug candidate for treatment of human colon cancer.