This is a subject that's near and dear to my heart, so here goes ....
Console:
Unfortunately, console users' options are limited (and they depend on what console you want to use). There are third party controllers out there that will allow you to set a button to "turbo" and have a "hold" function, allowing you to press the button once to turn the turbo function on and a second time to turn it off. For DA:I, this means that you could theoretically pull the trigger once to start attacking and then not have to pull it again until you want to stop -- no mashing or holding down needed (the controller would do the "mashing" for you). I know that a company called Hori makes some controllers like this for both the 360 and the PS3. Not sure about the newer consoles.
Edit: Forgot to mention that you can also buy custom-made controllers for just about any console (including Xbox One and PS4). You can design your own controller pretty much from the ground up, including swapping button functions around. Some custom controller makers also offer great accessibility options for disabled gamers, like foot pedals that will simulate the button/trigger of your choice. Custom controllers aren't cheap, though ....
PC:
PC players have a lot of options for workarounds. There are some great third party programs out there that will let you alter how a game is controlled in almost any way imaginable. I'll be using a controller on PC and a program called Pinnacle Game Profiler to alter the xinput and allow me to remap controls (not a fan of trigger for basic attack, so I'll be using another button instead). You could use Pinnacle or a similar program to more or less simulate the auto-attack we had the in first two games. The only difference is that you'd have to press a button once to start auto-attacking and a second time to stop (similar to how the controllers mentioned above work). Still better than mashing/holding.
There are similar programs that can give you the same functionality on a mouse or a keyboard (press once to start attacking, again to stop). There are also mouses like the Razer Naga that give you mappable buttons on the side, if you'd prefer to use your thumb for attacking, rather than your pointer finger. The Naga also comes with software that will allow you to program a press-to-start/press-to-stop auto-attack macro.