What eres command does is saves extra space on your GDDR when you first run the program and it creates the DAG. So -eres 3 loads the next 3 dags space difference. It does this because usually when there is a new DAG created and you need to rewrite it on your GPU, sometimes a crash can occur. So why the default is -eres 2. So it loads 2 extra DAG space onto your current dag.

When people started to run out of space, they changed the -eres to 1 or 0 because it loaded less on the GDDR and managed to get mining started. However it was only a temporarily fix. So -eres 3 will run out of space earlier than -eres 0 which doesn't preload any extra DAG space.

Let me know if this makes sense, its really hard to explain.