First off, I want to commend you for taking the initiative to actually write a story, especially when you have school and other things going on in your life. More and more young writers are starting to show their work, which is amazing :) I'm one of them!
Read the story now
Your book has great potential and the plot is interesting. The story starts straight off with action and an interesting character. I did wonder at first if this was a human version of a My Little Pony fanfiction, because there is an episode in the show about a crystal kingdom where a crystal heart ensures harmony, and a stallion called King Sombra has stolen it. If it's not your intention to write a fanfiction, I'd suggest changing King Sombra's name to avoid confusion.
In terms of grammar, there were a few times where you changed from past tense to present tense, and at times I wasn't sure which tense you were meant to be using. There was also some punctuation mistakes in the dialogue such as spacing, etc, but those issues can be easily fixed.
I found myself wondering about Annabelle's family. Why are they so poor? Where is her father? I was curious about her backstory but I do realize it's only three chapters in and that you might add that in later on, which is totally fine. I also wondered who Jakeem actually was--is he her friend, her crush, or her boyfriend? What does he look like, exactly? Adding character descriptions into your book really helps with imagery and getting a sense of the characters' personalities.
I also found myself wondering what time period the book is set in. It says that Annabelle puts on a T-shirt and shorts for bed, but before that I thought that it was set in medieval times considering they were living inside a kingdom and she worked at the palace. Depending on whether the setting is an alternate fantasy world or if it's set as a fantasy story in medieval times, perhaps you could add some distinguishing features so the reader can imagine it right. If it is in medieval times, Annabelle wouldn't be wearing a T shirt and shorts (a simple dress if she was a peasant, for example).
One other thing--this book is written in 3rd person, but that doesn't mean you can't show your character's emotions and feelings! Adding in little paragraphs of what is going on inside the main character's head can really help the reader engage with the characters and feel hooked to the story.
I know I wrote a lot, but I really think your story is so cool. Keep writing it!! You'll just keep getting better and better :) -from a fellow young writer