I shall be arriving at San Francisco international Airport Terminal from Singapore. After that I have to fly Portland from International terminal 3. Do I need to clear my immigration at SFO?

The Most Reliable answer, in this case, comes from the international flight airline you are flying, instead of a bunch of SFO customers "yes" and "no" guesses. This is, if a short time transfer to domestic in confusing signage SFO, to important possibly for customer guessing. Trust your international airline carrier.

It used to be that you only cleared customs when you deplaned and left the trade free zone. But it has been a while. Unless it has changed, a transfer to another international flight w a gate in same customs free area does not require you to go through customs..

Yes

Yes because you have to do immigration at the first port of entry, which in this case is sfo. Also note that terminal 3 is a domestic terminal.