AS3 Instantiate: Load a MovieClip from the library with addChild

Create a movieClip on your stage and name it mcDot.
Delete the mcDot from your stage.
Open the library panel, right-click the mcDot clip then select properties:

addChildMenu

Under the "Linkage" category check "Export for ActionScript"
Give the class the name "Dot". Make sure it is capitalized.

addChildProperties

Actionscript:

var dot:Dot= new Dot;
addChild(dot);

dot.x=stage.stageWidth/2;
dot.y=stage.stageHeight/2;

MovieClips: Position Movie clips with Actionscript 3

Create three movieclips on your stage.
Give the first an instance name of mcCenter.
Give the second an instance name of mcBottomLeft.
Give the third an instance name of mcTopRight.

Actionscript:

mcCenter.x=stage.stageWidth/2;
mcCenter.y=stage.stageHeight/2;

mcBottomLeft.x=stage.stageWidth-stage.stageWidth+mcBottomLeft.width/2;
mcBottomLeft.y=stage.stageHeight+mcBottomLeft.width/2;

mcTopRight.x=stage.stageWidth-mcTopRight.width/2;
mcTopRight.y=stage.stageHeight-stage.stageHeight+mcTopRight.width/2;

Example:

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