From what you say - virtually none.
Your example shows how important it is to update your will regularly (or at least review it).
If your mother were supporting you, you might be able claim that you were a dependant, and could reasonably have been expected to be provided for in the will (and contest it on these grounds).
You cannot contest a will purely because you thought the person should have left you something in their will.
My advice is to check the will out, that it has been completed correctly (witnessed etc) - if you can show there is a defect which invalidates the will, then you could have the will declared invalid. This would result in your mother effectively dying without having made a will (intestate), and her estate would be shared equally between her three children.
Bear in mind that if you did find a technical reason to void the will, a court may well decide that the will contained her wishes, and should not be ignored for a minor mistake.