There's something very deep inside me that tells me that beings with way more than the usual amount of legs are to be feared and I trust that part of me.
And that's it. There is no number three. You only need to learn to identify those two, because no other spider's bite is medically significant in the US unless you happen to be allergic (which some fraction of people are, to be fair).
Half your work is done because you probably already know what a black widow looks like; they are quite distinctive. (They also tend not to hang out in human living spaces like brown recluses do. Black widows prefer garages and barns.)
Both species are quite shy and rarely bite people. Most bites happen when the spider gets trapped between clothing and skin and it bites because it thinks it's about to be crushed. Neither species will chase you or jump on you.
There are spiders that will jump on you. They're called jumping spiders. They are harmless, quite cute, and make good pets. Unlike most spiders, jumpers have excellent eyesight and they will chase a laser beam like a cat.