This paper proposes three-phased traffic conditioner (3PTC) to be installed at edge routers in Differentiated Services (DiffServ) networks. 3PTC ensures that Assured Service (AS) flows are supplied with the throughput assurance, which stems from alleviating the impact of the size of TCP reserved rate, UDP/TCP interaction, Round Trip Time (RTT) and number of microflows. 3PTC is composed of token bucket phase, writing probability (WP) calculation phase and queue management phase. Computer simulation results show that 3PTC guarantees throughput assurance and provides end users with expected service levels.