Soft grippers have shown promising performance in safe and adaptive grasping tasks. However, they often suffer from limitations in grasping force. To address this challenge, this paper presents a novel pneumatic three-finger soft gripper to achieve robust adaptive grasping. The gripper consists of three identical fingers, each containing a pneumatic bending soft actuator and a pneumatic lateral soft actuator. The bending actuator features a tilted pneumatic network structure, which provides superior bending performance compared to traditional vertical pneumatic network structure. The lateral actuator is equipped with three deflection chambers at the finger root to mimic the lateral motions of a human finger. Kinematic and static models are established to predict the bending angle and grasping force of the soft finger under pressurized air. The performance of the proposed soft finger is analyzed through finite element simulations, and the effect of the chamber tilt angle is also examined. The theoretical and simulation results are compared to verify the validity of the analytical models. Finally, the proposed soft gripper is fabricated by 3D printing and molding. Experimental results show that the gripper is capable of grasping various objects of different sizes, shapes, materials, and weights, and can perform dexterous manipulation tasks, such as cap unscrewing. The proposed soft gripper exhibits significant potential for applications in robotic robust grasping tasks.