For practical purposes there is almost never a reason to start a bow intentionally to make the lower limb longer(it can be done, as an after thought in an emergency though, in certain situations). If the wood's quality is evenly resistant to bending the whole length, the longer limb will automatically bend further and be weaker. Not what you are after in the lower limb.
3 under means the centre of the pull will be lower on the handle than it is on split finger, when drawing . The tiller should look even in a drawn bow, no matter the style of limb design(maybe a little stiffer in the lower limb, which is prone to take a more strain). Where you place the hook on the tillering tree for 3 under needs to be lower than where it needs to be for a split draw.
