I certainly wouldn't call her a beginners horse, but that's me. There are people who have plenty of luck with green/young horses, but they're usually 100% under a trainer.
You can't tell much from the picture you've provided, but her legs look crooked. She's got a weird neck, but I can't tell exactly what's going on there. Looks like she's long through the back and her shoulder's steep.
I can't tell you for sure unless you provide a photo of her from the side.
Plenty of good dispositioned 3 yr olds are fine for beginners. Our mare lady was started as a lesson horse at 3 yrs and was great with kids. I think she's a beauty and will certainly fill out as she gets older.
I don't think anything is wrong with her neck, I bet you anything it's just her markings. My mare has markings like that on her neck, and if you look at it a certain way it makes her neck look goofy, but from any other angle it's a perfectly fine horse neck. =]
I think she's really cute, and as PaintHorseMares says, there are tons of 3 year old horses with enough brain for beginners. =]
I still think it has a lot to do with the markings making it stand out, as well as the way the horse is standing and holding her head. I stand by my opinion that her neck, from the pictures provided, appears just fine to me.