One muliticellular organism is a cube. each of its sides is 3 cm long. each of it cells is 1cm3,how many cells

You mean "one muliticellular organism is IN a cube"? If so, then:

The volume of the cube is 3x3x3 = 27 m3
27/1 = 27

There are 27 multicellular organisms in the cube.

A cube that is 3 cm on a side contains 27 cubic centimeters. (3 x 3 x 3)
If each of the cells is 1 cubic centimeter (and this is truly a cubic cell and a cubic organism) then the organism will contain 27 cells. <== Answer.
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I hope this is helpful.