The issues of common ground with voltmeter and ammeters
Question
How to wire up 3 sets of cheap volt ampere meters for the corresponding 3 power units 3.3, 5V, and 12V, of the ATX 12 PSU, taking consideration that the 3 ATX powers have a common ground.

Answer
Firstly, let us look at the user guide's wiring diagram.







Secondly, there is also a wiring demo video in the product, but I found it not that worth watching.

Thirdly, let us make sure if we can connect all three together with correct grounding,  with no risk of something melting down or blowing up.
The voltage meter and ampere meter are in the black box, so we need to read the wiring diagrams and reverse engineer/image what is inside the black box.  The following product sheet (Ref 5) includes the most detailed wiring diagrams.



Now let me see, according to Thevnin's Theorem, what goes in at black, -ve terminal of a PSU must come out at red. +ve terminal of the same PSU.  In other words, even the three PSUs have grounds connected, the amount of current coming out of the positive side of a PSUPSU must come back with the same quantity at the -ve side of the same PSU, ...  So I think it is 99.9% OK that the grounds of the 3 ATX's 3V3/5V0/12V can be (or already) connected together.
Warning: I am just a friendly hobbyist.  No guarantee no nothing won't melt down or blow up.
/ to continue, ...

References
(1) DPM/CMU DIGITAL DUAL 100V 50A Voltmeter Ammeter with shunt (OP's VA meter)
(2) VA Meter Productsheet - AliExpress
(3) VA Meter Wiring Demo Video (not that worth watching
(4) TaoBao Volt/Ampere Meter Catalog (in Chinese)
(5) DC0-100V 1A 10A 50A 100A LED DC Double Display (Voltage/Current) with fine tuning option, and very detailed wiring diagrams)
(6) Kirchhoff’s laws, Thévenin’s and Norton’s theorems - Eugeniy E. Mikhailov, The College of William &amp; Mary