You're going to have to leave your 2D AutoCAD thinking behind. While you CAN have 'static' dimensions in ARCHICAD, which are stand-alone graphic elements, the dimension tool is best used to dimension a building (or graphic) element - in which case it is 'associated' with that element.
If your 3'-0" dimension is of a 3'-0" wall ... you would stretch the wall to be 3'-3" for the dimension to reflect the new length. (In Revit, you could edit the dimension and then the wall would be adjusted to match the dimension. ARCHICAD doesn't have that capability.)
The pet palette will let you drag and move the dimension, change the leader line length, and more. It is all in the Help with illustrations.
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