This is a minor niggle of mine, but when looking for a cisco layer 3 switch visio stencil the collection of stencils and images over at www.cisco.com/web/about/ac50/ac47/2.html does not include a Layer 3 switch.
So over the years I have just used this image
It is actually classed as a 5500 Family but I have been using this image as a layer 3 switch in all my visio diagrams for many years.
Happy network diagramming!