Just 3rd and 4th will share the same cable/linkage, so you might be in luck. If you have to replace the box, consider before you start total cost of repair. What miles are on the clutch/flywheel. Obviously you are going to tackle it yourself, so a secondhand Haynes manual off E-Bay (no good going mad) might help.
Can you hold it in 3rd and jump 4th straight into 5, it will work out cheaper if its not external.
4 good forward gears, plenty for any car ................