3A is the internal current limit of driver chip. but its continuous rating is 1.5A (2A peak) per motor. But it can only provide current provided by upstream fuse, battery, etc...
It looks to me like there are two separate thermal fuses one for each of the two motor driver chips (Outputs AB and CD). Not sure what current they trigger at though....
I really need to open an EV3! Have you made any photo of the main board?
Not opened the EV3 up, but the Lego schematic seems to show 2 fuses for the two drivers.... But the scan quality is very poor so can't read the part numbers and other details...
I opened an EV3... I didn't want to take the risk of breaking something, so I didn't go too far away, but enough to see the fuses. Indeed, there are 3 of them, all rated 1.1A. So maximum available current is 1.1A (permanent, something around 2A for a short time) for 2 motor ports.