Ok, so - very subjective answer: as absolutely devastating the Israel-Gaza-West Bank war is, I don’t quite believe it could start WW3. @Fullofsurprises put it well here:
Iran has demonstrated before that it is reluctant to join in and none of the other Arab countries will, apart from perhaps some Syrian elements, although Syria itself is badly fractured and weakened by internal fighting and they probably won't want Israeli retaliation.
I’d also add that the Israel conflict has been going on for a long time, the various countries have had time to develop strategies to keep the world from falling into WW3 if and when the terrible situation there explodes.
Ukraine on the other hand - that I feel could start a chain reaction that could lead to a World war. Why? If Russia had succeeded easily in conquering Ukraine (which it obviously expected) a lot of people thought it would have turned its eye to other countries that belonged to the former USSR, especially ones that could form a buffer to the west. Estonia, Latvia and especially Lithuania, which would allow a direct connection to Kaliningrad oblansk, come to mind. If you look at a map you can immediately see how that might make sense with Belarus and Ukraine under Russian control. All three of these countries are NATO members, so if Russia attacked all NATO Allie’s would have to respond. We see how that could easily spread to a WW3…
That’s the reason all the Western countries pour so much aid into Ukraine. At least that conflict costs us only money, not bodies. If Russia takes Ukraine - who knows what would happen next.