The processor in a sun3 is a Motorola mc68020. On reset, the vector base register is set to 0x0. The first vector is the reset vector which provides two 32 bit words.
0x0 - initial interrupt stack pointer - 0fe60c00 0x4 - initial program counter - 0fef00eeJust how 0fef00ee ends up being the ROM is unclear. The sun3 manuals say that the board fires up in "boot mode" which can later be switched off by writing to the "System Enable Register". All I have yet seen written about boot state is that all supervisor program fetches take place from the EPROM.