Hi Dave, and I hope everyone will read this
This will be the second time in a month I'll have made a post that resolves a question like this using the SEARCH function of the library part dialogs. This is just basic functionality that everyone should know.
As Dave noted, the current US triple slider unfortunately forces all three sashes to be the same size.
But, entering Slider in the Window Settings dialog provides a list of downloadable window objects... and I've highlighted a triple slider in this screenshot. Note the globe at the lower right corner indicating that the object is not in a loaded library - but on the internet.
Clicking the Download and Embed button does just that - downloads the library part (and any macros required for it to function) and places it in the project's Embedded Library, ready to use (and in fact, the default object for the current tool).
Here is the Window Settings dialog after the installation and setting the window to be 8' wide with two 2' sliding sashes and a 4' fixed center:
You'll see the correct appearance in the preview in the top right of the dialog.
This is basically the window that Barry mentioned in his post that is in his AUS library.
(You'll notice that this window actually has more features than listed in the search / description - there is an optional transom where each of the three transom sashes can have an operating function. Not sure why Graphisoft did not mention that in the library part description.)
For every tool that places objects... always use the Search function if you are not finding what you're looking for