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

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.