Turn the question around. Ask yourself, what are the conditions necessary for this to become WW3?

First, at a minimum, you need more than one major power to become involved. For the purposes of this discussion I'll define make powers as The US, The EU / NATO (functionally indistinguishable for all practical purposes at this point), China, Russia, India.

Aside from the US, the only other major power with a serious interest in the region is Russia. You can vaguely make an argument for the EU, but they're too concerned with the war in Ukraine to really give a shit right. At best you might see some involvement from the UK, and they're no longer strongly aligned with or capable of influencing the rest of Europe.

So basically the only way this escalates beyond a limited regional conflict is if Russia decides to throw in. For Iran. While they're struggling to win a war in Ukraine that is currently cratering their economy and killing off an entire generation of young Russian men. Do you really think they have the slightest inclination to get into a shooting war with the US right now, with everything they're currently facing? I'm not even sure they have the logistics capacity to actually get troops to Iran right now.

But what if local powers get involved? They won't. No one in the region likes Isreal or Iran. They're both pariahs. No one around them wants to get involved in a war for either of their sakes.

Hell, they can barely even fight a war with each other. Rockets and bombing raids are the only real options they have, because there's a whole lot of Iraq in between them. At best the US could stage a naval invasion (something that Isreal has no capacity to do on their own), in which case what you're looking at is another Iraq style invasion followed by twenty years of bloody, costly occupation as they try to maintain order over a mountainous region containing a hundred million people.