How do I change and display a tooltip on a chart in c# when the mouse hovers over a bar?

I have a System.Windows.Forms.DataVisualization.Charting.chart and I want to show some information about a bar on the chart when you hover over it. But I can't see where to set a tooltip.

I can set this chart3.Series[0].ToolTip = "hello world";

but how do I pick up which x or y value I am hovering over in order to modify the text?