Distribut centre-Align

Abu,

Based on the said and the unsaid, the former especially this part,

"but sometime i use this in other shapes and there also i get this problem"

I have an interpretation different to that of Susumi and Mike.

The following way can be used for any quadrangle and triangle regardless of angles and sidelengths to make any number of divisions between two opposite sides, Smart Guides for snapping and the Line Segment Tool for drawing being your friends:

1) With the Line Segment Tool ClickDrag between the opposite end Anchor Points of the first one of the two opposite sides;

2) Repeat for the other one of the two opposite sides, making sure you ClickDrag the same way, thereby starting and/or ending at the same intermediate side(s) of the quadrangle/triangle (there being two for a quadrangle and only one for a triangle);

3) Hardly needed, but you can extend the length of each line from 1) and 2) to go beyond the the two opposite sides to ensure against gaps;

4) Select both of the lines from 1) and 2), or 3), and

4A) In Object>Blend>Blend Options set Orientation to Align to Path and set Spacing to Specified Steps with a number one less than the desired number of divisions (2 for 3);

4B) Apply Object>Blend>Make; now you can see the division of the quadrangle/triangle;

4C) Apply Object>Blend>Expand (optional unless you wish to have actual dividing lines, and especially if you wish to divide into smaller quadrangles/triangles);

If you need an actual division into smaller quadrangles/triangles:

5) Select each of the lines from 4C) and apply Object>Path>Divide Objects Below; this will create the smaller quadrangles/triangles.

Instead of 5) you can also use the Shape Builder Tool/Life Paint (you may get extra Anchor Points if you skip 3) if there are gaps).

Select sqaure

Object >> Path >> Split into grid: 3 column

Left-align the square and the left line.
Right-align the square and the right line.
Distribute horizontal space only on the 4 lines.
It is important to remove the square from the selection at this time.