Selection pressure probably doesn’t apply to evolution in the present tense as evolution is not a conscious entity. Survival of the fittest could have only a past tense application ie the cause of evolution traits surviving. In this case evolution doesn’t know if 3 arteries is good or bad it just happens those people have more kids and their kids have more kids until one day all the 3 artery people die because of some disease where having 3 arteries means you are more likely to die. This will leave 2 artery people living on and from their perspective looking back it’s then selection pressure.