This paper proposes a new multi-path position-based routing protocol for mobile ad-hoc networks and analyzes its performance. The proposed protocol is called 3DGMPR (3-Dimensional Grid Multiple-path Position-based Routing). It uses a logical 3-dimensional grid view of the geographical region. It allows the use of multiple celldisjoint routes for reliability. It supports adjusting the adesired level of reliability. Each node maintains a list of gateway nodes located in neighboring grid cells for efficient packet forwarding. The 3D-GMPR protocol has the following new features compared to other positionbased protocols: (a) it uses multi-path routing for reliability, (b) it uses efficiently pre-computed routing vectors avoiding path establishment and maintenance overheads and (c) it uses gateway nodes with low maintenance cost and distributed packet forwarding load. Performance evaluation results for the proposed protocol are obtained confirming its high reliability.