"I'm looking for a microcontroller which can operate at 50 Mhz clock having at least 3 external interrupts."

That (50Mhz )is a little bit on the higher side for a MCU ? More prudent would be to select something in the region of 20 to 33 MHz and with reduced number of cycles per instruction. The Cygnal ( now in a new 'avatar'), Dallas range are a few that comes to mind.

At such clock rates I do feel that the PCB design becomes a critical aspect particularly if you are planning to handle mixed signals - search with keywords PCB+EMI on this forum. Just thought will mention..

Raghu