The FL is caught between them. She has feelings for both of them and is conflicted, then she chooses the ML and ends up with him.

Would prefer if not a historical, but will take those as well.

Happy endings please.

Boys Over Flowers is currently the only one I can think that isn't historical.

For historical I'm pretty sure these fit those categories from what I can remember:

If I find anymore non historical ones I'll be sure to post them here for you since most of the ones I found are historical.

I just dropped it, so I don't know how it ends, but Cambrian Period fits all of your other criteria: action, romance, ML and SML are rivals on opposing sides + love rivals, FL has feelings for both of them, and it's not historical.