Hi,
I am designing a board with STM32L152VD LQFP100. From the datasheet pin definition table When I look for USART3 pins I find pin pairs 55,56 and 78,79 both for USART3. I am sure about pin 55 and 56 since only LQFP100 has it but not sure if pin 78 and 79 are connected to USART3 too.
Functionality can be multiplexed in/out by a number of pin options.
This allows you to escape the specific peripherals you want to use, out of the pin count device you are working with.
The parts generally share a common die (IC), they are just placed in different packages, and a subset of the pins bonded-out.
Review your part's "Data Sheet" to see the peripheral options on each pin, configured via the GPIOx_AFR register.