What does "bring to a simmer" mean?
Personally, I would argue that 2 and 3 are actually the same, and they are your answer.

If you heat a pan of water you'll notice the bubbles forming before the water is actually boiling, hence the talk of between not bubbling and full on roiling.

Also, when you're making your soup, it isn't pure water, so the boiling temp will not be a perfect 100 degrees C in any case.

So, I would say, that simmering is when you keep it just under a full boil. Watch what you're cooking, there should be gentle movement, but not a full roiling pan of whatever it is you're cooking.

To get something simmering away, you need to bring up to a full boil, then reduce the heat until you're getting movement, but not full bubbling.