This is a generalist answer not one specific to this phone!
One of two problems here, the circuit board still has dirt or water residue on it preventing the signals from the keypad from being interpreted properly (a short), or the keypad its self is not conducting the current from the button pads properly when pressed.
Because there are three keys not working then I would imagine its the first problem as the circuits for a keypad circuit is arranged in a grid and this look like the row conductor is not working.
Damage like this often occurs to mobile phones months after they get wet as they slowly corrode with the smallest amounts of water left sitting in between or under the chips on the boards. If the phone has had water spilt on it then it isn't usually a problem if dried properly, anything else though usually is more of a problem.
The rubber keypad can be removed and washed in warm soapy water. The underside of each button is a conductive rubber and needs to be free of any 'stickyness'. these are usually pretty robust. (Only the rubber bit with the printed numbers on it!)
Once cleaned pat dry, even put it somewhere warm dry, or use a hair dryer.. once your happy there is no water left in the buttons etc put it together and try the phone again. I suspect it wont fix your problem though but that's the easiest fix of the two. If you don't want to pay for it to be repaired then have a poke around as its not going to be much use as it is, you'll need to do a bit of investigative work.
Take the phone apart, look for a track on the main circuit board that goes to each area where the 789 buttons would sit, follow it from the buttons and eventually it will go to a black chip on the circuit board, somewhere between that chip and the button pads is your problem. Look how clean the circuit board is near that chip, its likely the 789 button track goes to one of the pins of the chip and it is shorting to one of the pins next to it. if you confident use a pin and gently scrap between the pins, check both sides of the board in the same area for anything dirt related.
If you can get hold of an air line blow under the chip where the tracks go, (make sure nothing is around that is loose on the board that can blow away)
Other than that without a friend with a soldering iron there's not a lot else you can do.
on Nov 19, 2014