I have been told that the new Stagetraxx 3 will not work on an Ipad using an IOS below version 11. I this true? My Ipad is using 9.3.6
That's correct, you need at least iOS11.
Can I still purchase a version of Stage Traxx that will work on my ancient iPad that maxes out at iOS 9.3.5?
No, version 2 is no longer available for purchase. You can only install it if you have previously purchased that version.