Is it possible to calculate the focal length of a refactor I built?
The focal length of a telescope is the focal length of the lens at the front of the scope. In your case it's either 50, 30, or 20 cm.

To magnify the image formed by one of those objective lenses, you need an eyepiece. None of the lenses you listed will work as an eyepiece unless you want a magnification of 2.5 (50/20).

Eye pieces generally have a focal length of 25 to 30 mm or less. To get a quality image, you should buy a used eyepiece.