First off: since there is only one 3-pin connector in most boards, the fans will have to be connected not to the board, but to the PSU itself. To do this you'll need a 3-pin to Molex adapter, which can be bought for a very cheap price. If your case comes with multiple fans, it will probably have at least two of those.
IMPORTANT: If you have more than one case fan, you'll need to equal the number of adapters and fans.
To connect these adapter to the fans and the PSU, simply plug one of your fans into the 3-pin end of one of the adapters, then plug the male Molex connector (the one with the four pins) of the adapter into one of the PSU's female Molex cable (the one with the four holes). Plug the other fan to the other adapter and plug its male Molex connector to the other adapter's female one, as shown in the picture below.
To install more fans simply connect the adapters to each other, regarding one of them should be linked to the PSU by the male end, and other should have a female end not hooked up to anything.
Some fans have a Molex power input instead of a 3-pin one, so you don't need any adapter to plug them to the PSU.
We have also what is called the "fan splitter". There are many types. One of them basically splits the 4-pin signal into two 3-pin signals.
This one is pretty easy, just connect the Molex end to the PSU and the fans to the two 3-pin connectors.
Another type is the one that splits one 3-pin signal into two 3-pin connectors.
This one will be connected to your motherboard's 3-pin header by one end, and to the fans by the others.
There is also one splitter which can be connected either to your PSU or your board. This one is the easiest.
If I commited any mistakes please let me know.