Low-speed model testing (LSMT) plays a key role in advanced multistage high-pressure compressor (HPC) design recently, due to this, employing low-speed large-scale compressor to conduct 3D blading design and detailed flow mechanism investigation is convenient and cost-saving. This paper is one portion of a whole LSMT project for the seventh stage of an advanced commercial HPC, and experimental investigations of 3D blading optimizations for LSMT were presented in this paper, consisting of overall performances for the compressor and stage 3 and detailed flowfield measurements including area traverse for rotor 3 inlet, stator 3 inlet and outlet, area traverse inside stator 3 passage, and static pressure on stator 3 blade surface. Compared with the datum compressor, revised rotor 3 is J-type and hub restaggered, and the improved stator 3 possesses characteristics of controlled camber angle, reduced leading blade angle, forward movement of maximum thickness position, and larger bowed-shape. Experimental results show that efficiency is improved by 1%, and total pressure rise for the compressor and the third stage is raised by 1.4% and 10%, respectively, while the stalling mass flow rate is maintained. The effectiveness of improved design methods is confirmed, and it is a guide for further blading design and optimization, furthermore, detailed flowfield measurements reveal the basic flow mechanism of all the improvement methods. Moreover, the results indicate that utilization of cfd code in the optimization procedure is promising, and the reliability and feasibility of cfd code are verified with the detailed experimental results.