HSSI is just a high speed serial interface which can support upto 45Mbp whereas, T3 rate is 44.736 MHz and E3 rate is 34.368 MHz.
The Cisco 3825 Integrated Services Router provides the following support:
* Wire-speed performance for concurrent services such as security and voice , and advanced services at up to half T3/E3 rates
* Increased density through High-Speed WAN Interface Card Slots (four)