While LGA 2011-3 and LGA 2011 share the same physical processor dimensions of 58.5 x 51.0mm and LGA 2011-3 and LGA 2011 share the same ball pattern pitch of 1.016mm, that's where the similarities end.
LGA 2011-3 CPUs are keyed differently than LGA 2011 CPUs, meaning you cannot put a LGA 2011 CPU into a LGA 2011-3 socket, nor vice-versa; They are physically incompatible. A LGA 2011 CPU will not fit in a LGA 2011-3 socket, and an LGA-2011-3 CPU will not fit in a LGA 2011 socket.
What's more, the CPUs that ride the LGA 2011 socket and the CPUs that ride the LGA 2011-3 socket do not use the same chipset. Even if you were an electronics engineer working for a big OEM like ASUS or Intel and you went and engineered a motherboard that has the chipset that LGA 2011-3 needs, and then you fabricated that motherboard with a LGA 2011 socket, there is no way that motherboard could work. The same goes for if you made a motherboard with the chipset that LGA 2011 needs and then tried to put a LGA 2011-3 socket on it. It also would not work.
And by the way, there is no LGA 2011-1 or LGA 2011-2. That's right: Just in case this wasn't confusing enough for you, Intel went and did that.
So in summary, LGA 2011-3 and LGA 2011 are neither compatible nor interchangeable. Thus if upgrading to an LGA 2011-3 motherboard, you will need to buy an LGA 2011-3 CPU; If you want to use a LGA 2011-3 CPU in a LGA 2011 motherboard then that will not work.