I think we need to have a little more info, or perhaps a rough sketch of what you are wanting to construct. If I understand correctly, you are wanting to build an open fronted box, ie, two sides, a top and a bottom and a back, and perhaps have the front glazed?
What sort of wood are you using?
I`ve been told to use a lap joint and then a mortice and tenon joint for the third joint. Is this the easiest way?
I wouldnt have said that a mortice and tenon joint is the easiest way, it could look very nice, but perhaps not as nice as joining the sides to the top/bottom with dovetails, but that is more difficult than the mortice and tennon joint.
There are a number of ways of doing this, the easiest way is to just butt the sides to the top and bottom and glue&screw them together and then pin the back on to hold the whole thing square.
A slightly harder way, but better would be to attach the sides to the top and bottom as above, then using a router, make a rebate around the inner edge of the rear of the top/sides/bottom assembly, just slightly deeper than the thickness of your backing material, then cut the backing piece to be slightly smaller than the rebated opening (to allow for movement of the wood in the sides/top/bottom) and pin in place.
One of the finest looking would be to dovetail the sides to the top and bottom, and then rebate the rear to accept the back piece, but dovetailing is not the easiest method to do.
I would say not to glue the back into the sides/top/bottom assembly if you are using real wood (ie not MDF or chipboard/plywood) as when it moves with moisture changes the back will not allow the movement and will cause distortions and even splitting. It is better to just pin the back in place with a few small brads (tiny nails).
That's just my rambling thoughts, a lot may depend on how comfortable you are with working with the brown stuff and what equipment you can get your hands on to help you.
Tim