<p>I use <code>public boolean mouseDown(Event ev, int x, int y)</code> to detect a click of the mouse.<br>
I can distinguish between the right mouse button (ev.metaDown() is true) and the left and middle.</p>

<p>How can i differentiate the left from the middle button?
Or if it is impossible with mouseDown, what should i use?</p>

This might do it:
http://java.sun.com/j2se/1.5.0/docs/api/javax/swing/SwingUtilities.html#isMiddleMouseButton(java.awt.event.MouseEvent)
Haven't tried it myself.