Adapters rated for 500mA usually can sustain quite high inrush current — 500mA just means it can maintain 24VAC when supplying 500mA current. But the current and voltage output are on a continuous spectrum — if you start drawing 750mA, for example, it doesn’t just go down to 0 volt, instead, the voltage drops to something below 24V (maybe 22V), and to most solenoid valves 22V works perfectly fine. Of course at some point if a device is drawing a significantly amount of current, then the voltage drops quickly to an insufficient amount.
Another thing to keep in mind that if all valves open at exactly the same time, then the demand for inrush current is quite high. However, if solenoids open one after another, even with just 1 second of delay between them, the demand for inrush current is significantly lower as only at most one inrush current is present at any given moment.
The DC-powered OpenSprinkler is similar, except it actually does the ‘staggering’ automatically — even if you program it to turn on 3 zones at the same time, it will stagger them by 1 second apart. The reason is that DC-powered OpenSprinkler uses an internal voltage booster, to generate a high voltage that can engage the valve. If all three solenoids are turned on at the same time, the impulse voltage may not be sufficient to drive all of them. So staggering them ensures that the impulse only supplies to one at a time, thus reducing the demand. So basically the principle is similar to AC-powered (except on AC-powered there is no automatic staggering so the burden is all on the power supply).
I would say if you want to run more than 2~3 amp of total current you should consider using a relay board. It’s safer and isolate the control side with load side. The calculation from spec sheet often gives you a pretty high number, but in reality, the demand for current is not that high. Most solenoid valves, even rated at 24VAC, would work fine without any problem with something like 20VAC, which also means lower current than spec sheet.
I am not entirely sure how you derived the 3.5A number: are you saying that you are running 11 (3+8) zones simultaneously? That seems a bit too much. If you mean 3 run at the same time, but you have a total of 8 zones, then the calculation should be based on the 3 zones. I am pretty sure even a 750mA adapter would be fine for 3 simultaneous zones. If you do plan to open 11 zones at the same time, I would highly recommend using an external relay board.