Just follow and trace the line paths shown on the solution below.
Samsung GT-I5800 power button Switch Jumper Ways
samsung i5800 power on-off jumper ways solution
Samsung Galaxy3 I5800 Power Button Ways
Samsung i5800 On Off Key Jumper Solution
Just trace and apply a tiniest magnetic jumper wire on the spots shown of the above solution.
The highlighted components above is where the power switch is being connected, if these components were damaged it will result to power switch failure. You may need to check and then replace these components as well.
There is a risk that the power switch on Samsung Galaxy3 I5800 may sometimes damaged and the switch terminal solder pads will be lifted and found hard enough to install it back. If these happens you may need to strongly bond the power switch to its original position and do a apply a jumper wire on it is one of the option.
You can use the solution above to determine which particular components you can put and solder a jumper wire when needed.