Think of funner secrets that are the opposites of the real ones ("don't you dare say I'm not here") and pretend to get mad and get all dramatic when she says you're not. The kid is 3. It might even laugh. Or something....
Put a lock on it, or put it up higher.
take that up with your mom. With videographic evidence.
Not tell her anything.
she is not young she doesn't keep ur secrets now..it will b better that u don't tell her ur secrets..and keep ur phone away from her or keep a password..