If three priorities is too much, I'd recommend two. I actually prefer this approach to the three rule, but there's a trick to it.
This is the Fix/Schedule rule.
Whenever someone comes to you with an accessibility issue, you make a decision on the spot. You either fix it then and there or you give it a real slot on your schedule. None of that "we'll get to it this quarter" bullshit. If you decide to schedule it, you assign it to a specific sprint. You pucker up and make a real commitment.
That's the trick. Schedule means scheduled, not logged somewhere on any list.
Most teams log issues because it feels productive. You're capturing the problem, right? Wrong. All you're doing is deferring. The issue goes on that damn list, loses its context over time and everyone forgets about it.
You either fix or you schedule. If it's not worth scheduling, it's not worth keeping. Delete it. If it's important, it'll come back to you, don't worry.
There is a clear disadvantage to all this. It puts pressure on whoever's making the call to either fix or schedule. That someone needs to have enough knowledge to judge how serious this is on the spot. They also need enough authority to either pull it into a sprint or let it go.
Unfortunately, that's not always one person. This leads to meetings and postponing stuff. Which is why it's not my preferred substitute for the backlog either.
But hey, if you can make it work, it is a pretty decisive system.