Can I substitute 3A diodes in a plug in power supply for 2A 50V diode? Should I replace all 4 in the bridge? It's a 15VDC 1300mA power supply.
No problem replacing the 2A diode with 3A if the PIV rating is the same or greater as in the original, 2A 50(piv). You do not need to replace all four, however two or more diodes may be bad in that bridge. The safe bet would be to do all four.
As long as the replacement diodes have at least the same voltage and current rating it should be fine. Diodes have other parameters too, but in this case they won't matter. It will be fine to replace the 2A 50V diode with a 3A 50V (or 100V or even 400V) diode.
You only need to replace the broken one. There is no issue of matching in this case.
It's safe because the bridge rectifier only sees rectified line frequency. If the diode was being used elsewhere (a high-speed rectifier in a switching power supply, for instance) other parameters like capacitance and reverse recovery time become critical, making substitution much more difficult.