I have an old-ish motherboard with a single 4-pin PWM header and two 3-pin headers. Unfortunately, the 3-pin version of the fan I want is no longer available I'm considering just buying 3 PWM ...

The world of electrical connectors is a deep and mysterious place. The first step is going to be figuring out the center-to-center pin spacing... this might be metric (1 mm, 0.5 mm, etc.) ...