Your motherboard probably has more than one 3 pin fan header. Sometimes they're kinda randomly placed it seems.
If not, yeah, you can get an adapter for a 4 pin source (it might even come with one)
Or, you can just plug it in and use it right away. Keep your case open and if it doesn't spin up, just turn your computer off. You wont kill your processor in the time it will take to turn the computer on and off because the heatsink only really needs the fan to get fresh air to run past it to cool it down once it gets hot. It takes a while for the heatsink to get really hot, so a few seconds will be perfectly ok.
Just start it up and then if the fan isn't spinning, hold down the power button and it will shut your comptuer down in about 5 - 10 seconds.