move object within given perimeter only - actionscript-3

Question

I'm creating a game(not exactly) for kids. It's to illustrate we can make infinite triangles with fixed length of thread, pin board and three pins. Like below:
Students can move any pin. Doing so will create new triangle but the perimeter of the triangle should be of given length. I used the following code:
var xMax:Number = drawBoard.x + drawBoard.width;
var xMin:Number = drawBoard.x;
var yMax:Number = drawBoard.y + drawBoard.height;
var yMin:Number = drawBoard.y;
var isDragging1:Boolean = false;
var isDragging2:Boolean = false;
var isDragging3:Boolean = false;
drawBoard.x = xMin;
drawBoard.y = yMin;
var lines:Shape = new Shape();
addChild(lines);
stage.addEventListener(MouseEvent.MOUSE_MOVE, mouseUpdate);
stage.addEventListener(MouseEvent.MOUSE_UP, stopDragging);
pin1.addEventListener(MouseEvent.MOUSE_DOWN, startDragging1);
pin2.addEventListener(MouseEvent.MOUSE_DOWN, startDragging2);
pin3.addEventListener(MouseEvent.MOUSE_DOWN, startDragging3);
function mouseUpdate(evt:MouseEvent):void
{
if (isDragging1)
{
pin1.x = validX(evt.stageX);
pin1.y = validY(evt.stageY);
lines.graphics.clear();
drawLines();
}
if (isDragging2)
{
pin2.x = validX(evt.stageX);
pin2.y = validY(evt.stageY);
lines.graphics.clear();
drawLines();
}
if (isDragging3)
{
pin3.x = validX(evt.stageX);
pin3.y = validY(evt.stageY);
lines.graphics.clear();
drawLines();
}
evt.updateAfterEvent();
}
function validX(thisX:Number):Number
{
if (thisX < xMin)
{
return (xMin);
}
if (thisX > (xMax))
{
return (xMax);
}
return (thisX);
}
function validY(thisY:Number):Number
{
if (thisY < yMin)
{
return (yMin);
}
if (thisY > (yMax))
{
return (yMax);
}
return thisY;
}
function drawLines():void
{
lines.graphics.lineStyle(1, 0xFF0000);
lines.graphics.moveTo(pin1.x, pin1.y);
lines.graphics.lineTo(pin2.x, pin2.y);
lines.graphics.lineTo(pin3.x, pin3.y);
lines.graphics.lineTo(pin1.x, pin1.y);
lines.filters = [new DropShadowFilter()];
}
function startDragging1(evt:MouseEvent):void
{
isDragging1 = true;
}
function startDragging2(evt:MouseEvent):void
{
isDragging2 = true;
}
function startDragging3(evt:MouseEvent):void
{
isDragging3 = true;
}
function stopDragging(evt:MouseEvent):void
{
isDragging1 = false;
isDragging2 = false;
isDragging3 = false;
}
function DistanceTwoPoints(x1:Number, x2:Number, y1:Number, y2:Number):Number
{
var dx:Number = x1 - x2;
var dy:Number = y1 - y2;
return Math.sqrt(dx * dx + dy * dy);
}
It does the half part. I can move any pin and it will create triangle. But the perimeter is different everytime. I tried to restrict it by calculating the distance between three sides. The problem with that is, I can move pin anywhere which makes it difficult to get exact perimeter. As a result, it makes it difficult to move pins smoothly. So, my question is, is it possible to move the pins only to valid location instead of moving along mouse? Any help would be appreciated.