Strain of massive structures, such as highway pavement systems, varies in every direction and it contains all the significant information about the safety of the civil structures. In this paper, optical fiber Bragg grating (OFBG) based sensor assembly packaged in fiber reinforced polymer (FRP), named OFBG based sensor, was proposed for 3D structural strain monitoring. Its dimensions have been firstly optimized according to the host structure characteristic, loading condition and acceptable strain transfer rate; then the stainless annulus and FRP outshoot, which can reduce the stress concentration and strain transfer error, were assembly utilized to develop the proposed OFBG based sensor; a case study of the proposed sensor applied to monitor the 3D strain of various asphalt concrete pavement layers of the highway system under different static loading conditions was presented in the end. In comparison of the in-situ test and simulation results, the proposed OFBG based sensor assembly monitored the 3D strain distribution of the highway pavement system effectively, which proves it is a promising solution for 3D strain monitoring in structural health assessment.