Quantum cascade lasers (QCLs) are relatively new sources of mid-infrared radiation (between 2.5 m and 25 m), and are very well suited to the application of in-field trace gas sensing, mainly due to their superiority of being robust, compact, wavelength-versatile, narrow line width and low power consumption. All these advantages make the laser absorption spectroscopy based on QCL light sources become one of the most popular technologies for the quantitative chemical detection in a variety of fields including atmospheric environmental monitoring, chemical analysis, industrial process control, medical diagnostics, security or bio-medical studies, etc. In the present work, a highly sensitive mid-infrared gas sensor employing a single continuous-wave distributed feedback QCL and an astigmatic multi-path optical absorption cell is demonstrated for the simultaneous measurement of atmospheric carbon monoxide (CO), nitrous oxide (N2O) and water vapor (H2O). By combining with an adaptive Savitzky-Golay (S-G) filter signal processing algorithm, the detection sensitivity and spectral resolution of the QCL sensor system are significantly improved. Compared with the traditional wavelet transform based signal de-noising technique, the developed adaptive S-G smoothing filter shows obvious advantages in terms of computational efficiency and selection of the optimal filter parameters, namely only two filter parameters (the width of the smoothing window and the degree of the smoothing polynomial) need to be considered. Currently, the QCL sensor system is estimated for the long term measurement of ambient air in laboratory environment. The results show that measurement precisions of 8.20 ppb (1 ppb=10-9) for CO, 7.90 ppb for N2O, and 64.00 ppm (1 ppm=10-6) for H2O at 1 s time resolution and 1 atmospheric pressure (atm) are obtained by using the quadratic differential detection scheme, which can be further improved to 1.25 ppb (for CO), 1.15 ppb (for N2O) and 35.77 ppm (for H2O) by increasing average time up to 85 s, respectively. On the whole, the QCL sensor system has significant features of portability and low-cost, moreover, it can be easily modified for the real-time analysis of other gas molecules through the choosing of corresponding QCL light sources. The QCL gas sensor can be widely used in the field of atmospheric chemistry and other applications. Future work will focus on H2O induced broadening coefficients for CO and N2O transitions near 4.57 m, which will be updated for the developed multi-species QCL sensor system, thus resolving the influence of water vapor broadening effect and achieving the measurement of gas concentration in a high humid environment with sub-percent precision.