Lucky man, having three refractors!.
To undesrtand which scope is best depends on what you want to look at. Refractors are generally good for the moon & planets. To calculate the magnification, just divide the focal length of the eyepiece into the focal length of the scope.
For distant objects , you need higher magnification that will allow you to see, say, neptune's disc and will also seperate double stars. So I'd use your 1500mm scope with a 10mm eyepiece, which is 1500/10 or 150x magnification. However, high magnifications with high focal lengths narrows your field of view, so objects cross the field (due to the Earth's rotation) seemingly quicker. If you're lucky to have a compterised Go-To scope, they can track objects on their equitorial mounts with servo motors. Other wise, you'll have to do it manually.
For more general star gazing, moon & planets, use your 500mm scope with a 25mm or 32mm eyepiece, which gives you between 500/25 or 20x mag and 500/32 or about 15x mag which should be enough to give you great views of the moon, show the moons of jupiter and saturns rings. And the field of view is nice and wide, meaning that you won't have to adjust the scope as much to track objects.
Longer focal length + shorter eyepiece focal length = higher magnification = narrower field of view
Shorter focal length + longer eyepiece focal length = lower magnification = wider field of view
You have three scopes to play with, so I'd just fiddle around until you find which best suits you. I'd go for the 500mm scope to look at the moon and maybe your 1500mm scope for more distant objects. A good selction of eyepieces would be say, 5,10,20,32mm which, when used with both scopes, will give you a good range of magnifications and varying field of views. I generally use longer focal length eyepices to initially locate planets and then pop in a 5mm eyepiece to boost the magnification.
Use longer focal length eyepieces with the 500mm scope to provide a wide field of view to observe nebulae and galaxies. Unfortunately, unless you are lucky to live out in the countryside away from light polluted skies, deep sky observing is difficult.
Here's some good sites to visit:
http://stargazerslounge.com/
http://www.popastro.com/