@raenndrops will be able to answer this better than I can......but this is what I have gathered from it.
The mobility rules have changed this year and technically yes a L3 is now supposed to enter Xcel at the Gold level. A lot of people think that is a bit extreme, given that certain skills like the silver vault are actually a step up from L3. There are some exceptions about score out meets, etc but I don't think that would apply to L3 since gyms don't have to score out for L3. However, she can be petitioned into silver, and I think that all is requires is an email sent in with the info and reasoning. Raenn will be able to tell you for sure, she's been a huge help to a lot of us as this also threw us for a loop this season since my dd who was going to compete gold, had one L5 score out meet this summer and her coach moved her up to L6. Later we figured out that since it was a score out meet, she could have technically stayed in gold without petitioning but coach has chosen to keep her where she is for now.
Attached the mobility chart