Well there are two aspects of getting it to work - the input and the UI
The input is easy, use the mouse pick event to check if the mouse is over the Button.
The UI could be done with any UI plugin or 3D element (all you have to do is test that the mouse over is the correct gameobject and then display the tooltip via the plugin or via gameobject activate (if you had 3D text already created with the tooltip parented to a gameobject). The UI part is up to you.