I'm an adult, but still have 3 baby teeth because i was born with no adult teeth to replace them. is this common?

Not uncommon : Many people have the same condition. Its not a rare thing to see but its not the norm don't worry about it just keep your teeth nice and healthy.

Not that rare: Many people are genetically missing permenant teeth. Today we have implants, crowns, veneers, bonding, fixed and removeable bridges and orthodontics to correct most cases.

Seen regularly: We see this all the time. When we do, it usually is from 1 to 4 teeth. Most of these primary (baby) teeth eventually need to be removed as they become loose in adulthood, although not always. The good news is that nowadays, they can usually be easily replaced with implant retained crowns to look as good and be as healthy and long lasting as natural teeth. 95%+ success rate!