Estimation a traffic over the design period is one of the critical factors in pavement design. Weight-in-motion (WIM) sensors have become popular for weight measurements. A three-dimension glass fiber-reinforced polymer packaged fiber Bragg grating sensor (3D GFRP-FBG) is introduced for in-pavement WIM measurement at low speed. Sensitivity study shows that the sensor is very sensitive to the sensor installation depth and the loading position. The developed 3D GFRP-FBG sensor is applicable for most practical pavements and it also shows a very good long-term durability. Among the three components of the sensor, the longitudinal component has the highest sensitivity for WIM measurements. Field testing results indicated that the 3D GFRP-FBG sensor has a measurement accuracy of 90%, 79%, and 67% for longitudinal, transverse, and vertical components, respectively. Thus, it is recommended to use the longitudinal sensing unit if only one dimension of the sensor is to be further advanced for practice.