My son had had Speech Therapy until he was almost, 3.
He made great progress.
Enunciation... is also per age and development.
Some letter sounds or sound combinations, are harder to say and require different parts of the mouth all working in tandem with each other at the same time, plus with the throat, etc.
Some letter sounds are just using the throat. Or tongue. Or lips, etc.
Per my son, even after he finished speech therapy, he remembered what he was taught. And the Speech Therapist, along the way, also taught me about "teaching" him and we were also given handouts and cute tip sheets on it etc. So, "practicing" speaking was still fun, for my son. And he'd even tell me, what he was taught etc.
You do not have to read, constantly, for her to improve.
Doing that, sure she will burn out. Because it is not fun, anymore.
My son's speech therapy, was more play oriented. Not drills.
You don't have to use tongue twisters per say.
Just use word families. Or sound families.
ie: mat, cat, bat. or, smell, spell, bell. Or, shark, sharp, shape, etc.
Or use Rhymes. Instead of tongue twisters.
Or, simply ask your Speech Therapist, about ongoing practice.
Many 3 year olds can be hard to understand. Because of enunciation.
Whether or not they have had speech therapy.
So, ask the Speech Therapist.
Anyway, just Google search "tongue twisters for kids" online.