I have a 12yo with anxiety secondary to ASD though he was actually diagnosed with anxiety years before his ASD diagnosis. Kindergarten was a really, really horrible experience for my DS and caused lasting damage. If I could change anything we did as parents it would be to completely redo how we approached school. I'm not sure anything would have made it rainbows and unicorns, but having a diagnosis going into K would maybe have started us off on a better foot by helping the school create a more predictable and supportive environment for him. As it was, they blamed a lot of his anxiety behavior on "bad behavior", which he quickly absorbed as his being a bad boy and things went downhill from there.
I always recommend "What to do when you worry too much" --
https://www.amazon.com/What-When-Worry-Much-What/dp/1591473144 -- for parents of kids with anxiety. 3.5yo is a little young to read the book with, but you can get it now and familiarize yourself with the ideas and language of helping her deal with anxiety. Like a PP said, you don't want to focus on the anxiety too much by comforting her out of it whenever she's anxious, but instead give her tools to deal with it. And 3.5yo is not too young to start with that.