So, I'm trying to wire 3 5V WS2811s...
...So intuitively, it seems like I'd have to put 15V into the power rail.
Do you mean 5V?
But figuring out voltage in parallel is never ever intuitive,
Actually, it's no problem. ...Everything in your house is wired in parallel to 120V or 240V (through different switches & circuit breakers) and everything in your car is wired in parallel to 12 (through different switches & fuses).
Just make sure your power supply can provide (at least) the required current (Amps).