Improving load regulation on voltage regulator
Dropout voltage on the 78L33 is 1.7V.  You are right at the limit.  If the 5V drops just a tiny bit, that will be reflected in your output voltage.
The load regulation of the 78L33 is 60mV.  If that is too much for your application, you need a better regulator.
LEDs are current driven.  You ought to be regulating the current to each LED.  You need a constant current source for your LEDs to ensure each gets the same current.
Even perfectly matched LED currents won't guarantee perfectly matched LED brightness.  You'll either have to select LEDs that deliver the same brightness (test and measure LEDs for brightness) or you need to regulate the current with feedback from a light detector (photo diode or photo transistor or other sensor.)




If you just want a uniform brightness for lighting (eyeball uniform)  then controlled current with all LEDs from the same model ought to do.

If you need brightness matched for some kind of precision measurements, you'll need to go with current controlled with feedback.