Theft is simply the unlawful taking of another person's property without the intent to return it. This is the lowest in seriousness of the three crimes under discussion and carries the least sentence which usually depends on the amount taken.
What Is Burglary?
Much more serious than theft burglary is the unlawful entry into a building structure with the intent to commit a felony. There are usually two degrees, the first being a felony. Burglary is much more serious than theft particularly where the structure is inhabited.
What Is Robbery?
The most serious of the three, robbery is a felony and, in many states with 3 strikes, laws a strike. Robbery is the taking of property from a person with force. The force can be very slight to elevate the crime from theft to robbery. If you see someone with a dollar bill in their hand and you snatch it from them that is enough force to land you in felony land.