Why does the topic imply that the only possibilities are "Villain is 3-barrel bluffing" and "I should fold because I'm behind"?
Villain can show up with a value hand like Ax
or even Ax non-spades a lot of the time.
Adding up your "LAG" image and being so passive post-flop, he could easily believe you have a log of crying-call hands like pocket pairs or weak Ax. He also knows you don't have an 8 or a boat, and every check reduces your chances of having AK/AQ, so this is a safe value-bet from his perspective.
With over 3:1 on that bet you absolutely have to call. You are ahead of at least 25% of his range.