Oval designs are created with a specific length to width ratio. If you want to cut a stone to fit a 3 by 5 setting you need to use a design with the same l/w ration. For example an oval design with a 2 to 1 ration will not fit a 3 to 5 setting. This is assuming of course that you are doing meet point faceting.
Now with that said, you can take a design that does not have the desired l/w ration and using Gem Cad you can alter it to fit your desired l/w ration. The best person to consult on that subject is the Gem Cad author, Robert Strickland.