Is this a good gift for my girlfriend of 3 years birthday?

Very unique

After 3 years together, she might think any ring could be an engagement ring. If it isn't meant to be that, it could actually hurt her feelings. I would give her some lovely pearl drop earrings with a small diamond accent like the ones at the link. They are not too small but not too big and nice enough for a special occasion but would be okay for a business environment, too. She'd love them!! : }