I have a couple of question. I'm new to this so please bare with me. I bent my back rim, so my friend gave me a set of triple walls. But the front came with a flat axle. My questions are why are some of the front axle flat and can i use it on my bike its a 03 415 fsx it came with a 14mm round front axle. Thanks for any help you can give.

some front axles are flat so you can have 14mm axle in a 3/8" slot. chances are your front drop out are 14mm and it would not matter. your friend probly has 3/8" front drop outs and wanted to run the bigger axle.

If the front wheel has a 3/8" flat section on it you may need to procure a couple of those stepdown washers. They allow a 3/8" axle to fit securely in a 14mm slot.

the axle, i'm guessing is a 14mm one. it has a flat spot that is 3/8" so it will fit into a 3/8" slot. this was already mentioned.

I'm doubting that it's a 3/8" axle WITH a flat spot.

because it's a 14mm axle, you won't need a step down washer, they wouldn't fit anyways.
just drop the front wheel in your forks, it it doesn't slop around, run it.