Boosting the nonlinear conversion rate in nanoscale is pivotal for practical applications such as highly sensitive biosensors, extreme ultra-violate light sources, and frequency combs. Here, we theoretically study the enhancement of second-harmonic generation (SHG) in a plasmonic trimer assisted by breathing modes. The geometry of the trimer is fine-tuned to produce strong plasmonic resonances at both the fundamental and SH wavelengths to boost SHG intensity. Moreover, it is found that breathing modes show remarkable ability to augment SHG by increasing the enhancement area. In particular, these breathing modes ensure a substantial spatial mode overlap at the fundamental and SH wavelengths, resulting in further promotion of the SHG conversation rate. We envision that our findings could enable applications in nanoscale frequency converters with high efficiency.