As far as canned DC-DCs go, avoid cheap counterfeit junk, like everything that has a "LM2596" from the usual suspects like aliexpress or amazon. The only thing guaranteed is they will not meet their spec, and if there are electrolytic caps on them, you can bet they'll be garbage.
There are lots of inexpensive readymade products from trusted manufacturers...
If you don't make a pcb for your power supply, and you have 3 DC-DC modules with wires connected to the same input and output ground, then you have lots of ground loops with choppy current in them, which make nice loop antennas. Much better to put the 3 DC-DC modules on a PCB with a solid ground plane.
Input current is a square wave, so you need enough high-frequency ceramic caps there, and maybe a LC ferrite input filter to prevent the input current from becoming noise that is radiated by the input wires.
As far as output is concerned, that depends on the load, so you should look at each load in your design and consider:
microcontroller: wideband noise generator, doesn't care about noise on its supply.
opamp: unless feeding a low impedance load, doesn't make much noise, high PSRR ar low frequency, but low PSRR at high frequency
reference voltage for dac, adc, ratiometric sensor, etc: doesn't make much noise, but PSRR is essentially zero, noise on the reference voltage ends up in the output.
Then you partition your power rails into domains. For example, the MOSFET driver that draws 10ns 1Amp spikes (high HF noise) would not end up on the same +5V domain as the opamps that have low HF PSRR (thus high HF sensitivity).
Next, ponder which low frequency power supply impedance each domain requires. This will be low if it draws variable current and needs very stable voltage. But it can be a few ohms if it's an opamp that draws a few mA AC current and can tolerate the corresponding mV ripple due to its PSRR.
Next, all domains that can tolerate a bit of supply impedance are separated by LC filters with L being a ferrite bead. The bead should have good filtering at the offending frequencies, withstand the DC current without saturating, and not ring with the caps. The latter will usually require an electrolytic cap or a resistor in series. Get your ferrite spice models from murata.
So in the example above, the FET driver has its local decoupling cap which handles its HF current in a short local loop, then ferrite to main VCC rail, which has caps, then ferrite to opamp, which also has its local cap. So for a few cents worth of parts, you get a 5th order lowpass filter between the FET driver and the opamp, which means you probably don't need an extra LDO for the opamp.
If you use a bit higher voltage switcher followed by a LDO, you will get low ripple at low frequency, which opamps mostly won't care about. However at low dropout voltage, most LDO's PSRR suffers (check datasheets) and the input-to-output capacitance of the pass device also increases at low dropout, which means the HF ripple and spikes from the switcher go straight through the LDO. So, post-switcher LDO is a nice solution that works, but only if you actually need it, and if it is done right, with a LDO that has meaningful PSRR at the dropout voltage you will use and at the frequency where it matters.
Also the LDO does not replace ferrite beads.
Now the most sensitive node in your design is probably going to be the voltage reference for the DAC, ADC, or something like that. And that also draws very low current, which means you can use a linear regulator from a higher voltage rail or a low noise voltage reference IC without worrying about output current and dissipation. And you can also put a nice filter in the reference's input supply, since it draws low current this time you are allowed to put a highish value resistor in series with the ferrite to really pimp the mid-frequency PSRR.
Also you should not get married to your rail voltages. I suppose you use 12V for the opamps. They'll run just fine on 9V or 11V, or maybe even 5V. So if you have a special opamp that needs a special low noise supply, but doesn't use much current, like the first opamp in your signal chain that amplifies the tiny input signal... Then you can run it on a 5V or 9V linear regulator from your 12V rail. High dropout voltage, thus high PSRR ; low current thus low dissipation and RC filters are allowed too, not just LC.