what differentiates in 2/12 way and 3 way speaker ?

Much too general a statement. Only worry about how a speaker sounds not how it gets there. 

what differentiates in 2/12 way and 3 way speaker ?

      It's just marketing!

the difference is 1/2 a way

Imagine a 3-way with 2 woofers.  If the crossover rolls off one woofer to mate with the midrange but rolls off the other at a lower frequency, it becomes a 2 1/2 way.

Much too general a statement. Only worry about how a speaker sounds not how it gets there.

      I agree!

My speakers are two way with 2 15'' woofers with a horn above said woofers. The crossover is 1000 Hz @ 12 dB per octave. It has three drivers but is still a two way speaker.

Mike

kr4 thank you for your explanation

Here is the layman's explanation as I see a 2 1/2 way.  Of course you have your tweeter.  Then you have two identical mid woofers.  One mid woofer reproduces high to lowest mid-range, the second reproduces only the lowest mid-range frequencies. 

But at least in my case, both the mid-woofers are exactly the same speaker models..

Earlier, I posted this too quickly and in error:

Imagine a 3-way with 2 woofers.  If the crossover rolls off one woofer to mate with the midrange but rolls off the other at a lower frequency, it becomes a 2 1/2 way.

Here are two correct alternative re-statements:

Imagine a 3-way with 2 woofers.  If the crossover rolls off one woofer to mate with the midrange but rolls off the other at a lower frequency, it becomes a 3 1/2 way.

Imagine a 2-way with 2 woofers.  If the crossover rolls off one woofer to mate with the tweeter but rolls off the other at a lower frequency, it becomes a 2 1/2 way.

kr4, my speaker have one tweeter, one mid and 2 bass identical, Ar my speaker the 31/2 way or 21/2 way ?

kr4, my speaker have one tweeter, one mid and 2 bass identical, Ar my speaker the 31/2 way or 21/2 way ?

Depends on the crossover network.  What speaker is this?

kr4, speakers are ML 60 xti.

"The Motion 60XTi measures 48”H x 11.4”W x 14.4”D, weighs 66 pounds, and, along with its FMT XT tweeter, has one 6.5” midrange and two 8” woofers in a three-way configuration. The tweeter hands off to the midrange at 2.2kHz, while the midrange and woofers transition at 400Hz. "

kr4, from audio advisor, it is a 21/2 way ?  

Low-Mounted Woofers Minimize Floor Bounce

Floorstanding Motion models utilize 2.5-way designs that feature woofers positioned low in the cabinet and close to the floor. This deliberate design addresses a phenomenon known as floor bounce where sound reflected off the floor (from drivers positioned high on a cabinet) and sound aimed directly at the listener interfere with each other. Without addressing these issues, listeners would experience bloated bass and massive increases and decreases in sound levels throughout critical midrange frequencies.