You have 3 beautiful dogs! I am so glad to hear that Molly has found a perfect home with you. It sounds like she was due for some love & attention.
I will be anxious to hear what the vet says about Molly's health concerns. You have received some good ideas from others on the reasons she may be struggling. The only thing I might add, is besides needing her teeth cleaned, her breath may be effected by the food she has been eating.
I believe a quality dog food (human grade) can make all the difference in breath. Some fresh veggies might help her too.
Glad you found YorkieTalk!
yorkiesmiles & Bubba
& Roxie