i have a couple questions about SPR

is SPR measured by the shortest stack involved in the hand?

If we play a three way pot on the flop, and we are 150bb deep, another opponent is 100bb deep and another opponent is 50bb deep, what is the SPR if the 50bb player goes all in, and the 100bb player calls? At this point two players are left in the pot: hero still has 150bb, and another player with 50bb left, how do we measure SPR here?

In a multiway pot, there are multiple SPRs imo. Although the effective stack is the smallest, you can't really generalize a strategy based on the shortest stack alone (or the biggest, for that matter).
The same principle arises pre-flop. You could be on the button with 100bb and the SB might be 100bb deep too, but the BB might only have 20bb and be playing a jam or fold strat, so you have to form a strategy/range that performs well vs both players.

P.S. If someone with 100bb called a 50bb jam on the flop. He's unlikely to be folding on any turn when the pot will be more than three times his remaining stack.
SPR "problems" would more commonly exist where one player has an SPR of <2 and the others have SPRs of 4+ or so. Playing those spots is particularly difficult if you're not able to close the action. e.g. the shorty donk-shoves the flop and you don't know if you should call or raise to 'isolate' him.