S.H.
At 3 years old.... they are developmentally changing a LOT including their emotional development... which is NOT fully developed yet.
At this age as well, they develop "fears." It is developmental based and even the more confident and independent child gets that way.
Main thing is don't force it. It will make it more stressful. Nor "compare" her to others... this will not work.
Just assure her, let her know you "understand" even if it seems petty to you.
Next, when I was pregnant with my 2nd child, I spent TONS of time on my oldest child, prepping her for her baby brother. I spent "my" pregnancy prepping HER... and talked to her about it, let her bond with my tummy (her brother), explained in simple terms that I will get tired and hard to move around etc., but that SHE was my first baby and I loved her and will always love her. THIS helped her adjust and transition for my pregnancy and her upcoming baby brother to come home.
"Confidence" or not, is not something that will always always just be there... it ebbs and flows. Talk with her too, about how she feels. Let her say anything she wants... and just console her. My daughter NEEDED to talk story about things... anything. And it helped her feel centered.
But tis' the age.... my son is 3.5, but from about 3 years old he developed "fears' and its no biggie. It is just a developmental thing they go through. He is very self-reliant and confident, but he gets scared sometimes. Its okay. When our Aunty says he's a "scaredy cat", I do NOT allow that. It is just a normal developmental thing that occurs... and you just help guide them. They will outgrow it.
And, it sometimes means, that you keep on with their routines or amend them. Sometimes they may need other things to do... or outgrow certain things, or not like certain things anymore.
Or, did something happen at gymnastics that frightened her??? Ask her. Then validate her. Or maybe all of these "expectations" upon her... is too much right now?
A 3 year old is hard.... and they don't have mastery over their emotions. Help her to communicate her feelings... and let her know its OKAY.
All the best,
Susan