finding optimized size of something by changing three variables

Hi, so im basically trying to find the best suited pipe length for a flow and i need to change three different variables simultinaliously to find which variables work best together to optimize this. Can anyone help or suggest the best way to do this?

Thanks

[Ameer Hamza]

Can you show the objective function to quantitatively define the "best" combination of three variables? Also, see fmincon().