Three feet with an 8x10 puts you well into bellows-extension range.
A twelve-inch lens at 12" bellows is at infinity focus. No bellows extension factor needed.
A twelve-inch lens at 24" extension will cut your light transmission by two stops. Likewise, 6" of extension will cost you 1 stop, 3" of extension will cost you 1/2 stop, etc. Think inverse square law - the intensity of the light decreases by the square of the inverse of the distance between the film plane and the lens aperture diaphragm. Double the bellows, quarter the light. Just guessing, but an object at 3' will probably require at least a stop of exposure compensation. The rule of thumb I learned in school is that you do not need to compensate for bellows extension at distances greater than 8 times the focal length of your lens. You're safe at 8' and beyond with a 12" lens.