As an example, using a 50mm lens at infinity you will need 150mm extension to get 3:1 magnification. At this high magnification you will need to increase exposure substantially since you'll lose a lot of light. The working distance will be 67mm, which is the distance between the lens and the subject.
Please note that the actual magnification a certain extension tube or bellows extension causes is not that simple. The internal lens extension and the lenses specific magnification factor of a specific lens must also be taken into considerations to get the exact magnification. In this example that is not taken into account.
Light loss is according the following simple formula:
Ef = Effective f-value (that is what you get after adding extension tubes or bellows)
If = Indicated f-value (that is shown on your lens or display, the one that is set on the lens)
F = Focal length of the lens
Ex = Extension (total length of the extension tube or bellows)
As an example, with a 50mm lens and a 150mm extension tube you get 3:1 magnification and if the lens is set to f8.0 than:
That is the effective f-value used for the exposure, meaning you'll need to increase the light 16 times compared to not using any extension at all. That’s why ring flash or some other light source near the subject is needed, or you have to increase the ISO sensitivity. The drawback of that is that you'll lose details, so it is better to increase light than to increase ISO. Of course, you can decrease the shutter speed also, but that is not always possible and in fact it is not recommended at all since it may result in motion blur caused by subject motion or camera shake.