Easiest way I can think of:

function Start()
{
    var textMesh : TextMesh = GetComponent(TextMesh);
    var originalText = textMesh.text;
    textMesh.text = "";

    yield WaitForSeconds(3);

    textMesh.text = originalText;
}

Alternatively, set the textmesh's text to "" by default, then just get the textmesh component when your laser show finishes, and change the text to the right text

Edit (since you changed your tags to guiText):

function Start()
{
    var originalText = guiText.text;
    guiText.text = "";

    yield WaitForSeconds(3);

    guiText.text = originalText;
}

No OnGUI necessary

for 3d text to be occulted like any other surface : use this shader http://www.unifycommunity.com/wiki/index.php?title=3DText