Way 3 is the observation that everything we see in the universe is contingent = they are moved, changed and caused: they need not exist, but they do.
All living things die eventually - everything is contingent
From the observation that everything is contingent, Aquinas concluded that something must exist necessarily - there seems to be nothing we observe that explains why contingent things exist
P1: Everything in the natural world is contingent
P2: If everything is contingent, then at some time there was nothing, because there must have been a time where nothing began to exist
P3: Nothing could've come from nothing
P4: Therefore, something must exist necessarily, otherwise nothing would now exist
C1: We believe this to be God