Mobile video broadcasting service, or mobile TV, is a promising application for 3G wireless network operators. Most existing solutions for video broadcast/multicast services in 3G networks employ a single transmission rate to cover all viewers. The system-wide video quality of the cell is therefore throttled by a few viewers close to the boundary, and is far from reaching the social-optimum allowed by the radio resources available at the base station. In this paper, we propose a novel scalable video broadcast/multicast solution, SV-BCMCS, that efficiently integrates scalable video coding, 3G broadcast and adhoc forwarding to balance the system-wide and worst-case video quality of all viewers in a 3G cell. We study the optimal resource allocation problem in SV-BCMCS and develop practical helper discovery and relay routing algorithms. Through analysis and extensive OPNET simulations, we demonstrate that SV-BCMCS can significantly improve the system-wide video quality at the price of slight quality degradation of a few viewers close to the boundary.