I'm trying to put a PN532 breakout board, BLE "shield" daughterboard, and a PIC microcontroller all onto one board. The PN532 breakout board has an ADP121 (5V to 3.3V) regulator, the BLE breakout has a XC6204 (5V to 3.3V regulator), and I also have an AZ1117T (5V to 3.3V) regulator for a level shifter and a RS232 transceiver chip. I'm curious about what is usually done with multiple regulators with the same input and output voltage? Should I cascade them, or choose the best one and eliminate the rest?
Since each board was designed separately odds are that each regulator is rated only for the chips each board includes.
The first board might need some 100mA to work and have a regulator that can go up to 150mA, and so on. It's possible that none of the regulators you already have is capable of supplying enough current for all the chips you are planning to put on the same board.
First of all you need to compute the total power consumption of all the stuff you want to put on the board. You find the values on the chip's datasheets of course, use the higest value if you are not sure. The sum of all the currents needed by the various chips will of course be the max current the board will require. Add another 10 to 20% as a safety coefficient, then look the regulator's datasheets: probably none of them can supply that much current. Finally search for a "stiff enough" regulator and use it.
If you are simply packaging the three existing boards into a system, without modifying the boards, I would leave the existing regulators alone, and use the boards as purchased.
On the other hand, if you intend to design a new PC board incorporating the functions of these separate boards, then I would use a single 3.3 volt regulator, making sure it can supply enough current (and is on a suitable heatsink) for the full system.
As rightly pointed out by Vladimir Cravero, having only one 3.3v linear regulator is the best way to go ahead.
3 linear regulators will unnecessarily waste 3x the power of your input supply. Plus, if you need a 3.3v rail on your PCB or circuit separately (for supplying more ICs or sensors), extending a 3.3v line from one of the 3 regulators will not be a good idea.
so, how to decide the specifications of the single 3.3v regulator? Check the datasheet of all the sensors/devices you are using. Note down the peak current, Ipeak and peak power dissipation,P(peak) of the devices. Now select a linear regulator which can supply Ipeak current and P(peak) power in normal operating conditions (aka recommended operating conditions).