Rstofer, Nonsense. You don't need more than one DVM to measure three different simultaneous circuit currents or voltages, simply use one meter to measure each, one measurement at a time.
No, he's absolutely right and you could not be more wrong.
First of all, "simultaneous" means
at the same time, it does NOT mean "sequential". For simultaneous measurement, you need as many meters as you have measurements. By definition, you cannot take simultaneous measurements one at a time.
Measuring multiple voltages and currents
at the same time is a common thing, and often absolutely necessary. Not every set of measurements are so static that you can take the time to move around your meter. And measuring current with a regular multimeter means putting it in series, so to reuse the meter for a voltage measurement, you have to remove the multimeter from the current circuit, which changes it (since the DMM inserts some series resistance).
And this isn't advanced stuff we are talking about: classic beginner experiments involve things like varying the voltage through a resistor or diode and seeing how the current changes in response. For this to not be horrifically tedious, you want to be able to see the effects in real time. rstofer's BJT example is another classic experiment.
A frequent setup for many applications is four meters: input voltage, input current, output voltage, output current.
And besides, he didn't say "you need 3 DMMs right away", he said that you will
eventually need them.
Your advice to all doctors would be to use two stethoscopes at the same time to examine the two lungs of a person!
A more apt analogy, but of your silly advice, would be that an ER doctor only needs to have one bed, because they can just use it one patient after another, since the doctor can only see one patient at a time.
See how silly that sounds?