I hear this question a lot from new paddlers just getting into the sport. So you just bought your first paddle board and it came with three fins, or one fin, and maybe even five! Sounds confusing, let me see if I can help.
For example our 10' Cardiff Paddle Board is one of our best selling all-around models over the years. The board comes with three fins. The largest fin is the center fin and is 9" in depth (measured from the surface the board to the very tip of the fin when inserted). The smaller side fins are 3.5" in depth. So do you need to use all fins? The answer is no, and it depends on what you're trying to accomplish. You can get very technical and in-depth with fin science so for the sake of simplicity I will keep it pretty simple.
Remember this, fins or anything else attached to your SUP board that is under the waterline will cause drag. So the less drag the faster your board is through the water, and the more efficient it will be. So if what you primarily are doing is flat water paddling in calm conditions you can get away with using just the center fin. What this will do is reduce your drag, but also keep your board tracking (paddling in a straight line without having to switch paddle stroke sides often), adequately enough when your paddle speed and efficiency is the most important goal related to your fin set up.
On the other hand if you are a brand new paddler who may benefit from increased stability, and who is not necessarily worried about drag and efficiency while you're learning, you may opt to put all three fins in your paddle board. Also if you plan on doing some paddle surfing, having the extra side fins in will add to the performance of your board while riding the wave. When all the fins in the common "2 + 1" fin set up are in, the side fins add to the stability of the board and will also allow the board to track in a straight line better. The addition of the side fins will give the paddle surfer more traction and added drive on the wave. The downside to adding the additional side fins is that it will create a bit more drag.
Another thing to keep in mind is the longer and wider the fin equals more drag. But the upside to larger fins is more stability and better flat water tracking. Placement of the moveable center fin will also give you a different feel while you're paddling you local waterway. In general the further back the center fin is the better the board will track straighter through the water, and the further up you place the fin the board will be able to turn quicker, and or "pivot" better while surfing a wave.
So play with your fin set up and make some tweaks here and there as you progress. Fin science is pretty interesting stuff!
Any additional questions you may have, we are always happy to give you our tips and thoughts since we love this sport as much as you do!