If you actually look at the datasheet for a 1N400x diode, you'll see it's forward
drop varies from about 0.6V @ 50mA to about 0.8V at 250mA, so the output
of the 3 diode ckt will vary from about 3.2V to 2.6V with those loads. I'd go
with an LM1117, or other v.reg as mentioned, instead.