A multiplex reverse transcription polymerase chain reaction (mRT-PCR) assay was developed for the simultaneous detection of canine distemper virus (CDV), canine respiratory coronavirus (CRCoV) and canine influenza virus (CIV). These viral pathogens are all causative agents of canine infectious respiratory disease (CIRD). The sensitivity and specificity of the mRT-PCR were determined by comparing it to a rapid antigen test (RAT) or immuno-chromatography test kit and reverse transcription-polymerase chain reaction (RT-PCR) in the detection of CDV, CRCoV and CIV antigens present in 100 clinical samples (nasal swabs and whole blood samples) from 50 dogs with respiratory disease symptoms. This study revealed that mRT-PCR had almost exactly the same performance or results were almost 100% in agreement with that of RT-PCR and RAT both in terms of the assay sensitivity and specificity which was more highly evident in detecting CIV, CDV and CRCoV antigens present in canine nasal swab samples. Therefore, this assay could be a better alternative for the definitive and simultaneous ante-mortem detection of the three viral pathogens that cause CIRD by using nasal swabs.