Hard to say of the iPad will ever get 3D Touch. The regular 4.7" iPhone 6s I believe uses something like 96 sensors to detect the pressure (they measures the distance of the light coming from the backlight to the back of the display and detect the miniscule changes.)
A 9.7" or even the 12.9" would take a huge increase in the amount of sensors required.
Then there is the matter of the Taptic Engine that gives you haptic feedback when you Peek and Pop and do other 3D Touch functions. iPads have never had any kind of vibration motor for feedback (neither the old motor or the new Taptic Engine.) iPads are large enough that just using one probably wouldn't suffice (especially not in the 12.9") they would have to either make one large enough you would actually feel it, or add in 2-3. Both methods would require new space to be made to fit it/them.
All of that can be overcome with time and probably will be. But when is the big question. Currently, it's probably not cheap to implement it at such a large scale (9.7" and 12.9") right now.
It's comes down to of you want upgrade what they release later this month and enjoying the massive upgrade over the old iPad 3 and use the Pencil. Or wait another whole year for a very big "maybe" on 3D Touch in 2017.