[Mikeod]

It's really not difficult if you think of the roundabout like a regular intersection. You wouldn't make a left turn from the right lane at a regular intersection, right? Nor a right turn from the left lane. Approaching a roundabout you need to get or be in the proper lane for the direction you will go exiting the roundabout. Right lane for straight or right turn (first exit). Left lane for straight or left turn (third exit).

But the most important thing is to not enter a roundabout when another vehicle is already in it, even if the right (outside) lane is empty since you don't know where that vehicle is exiting. People entering the roundabout that way and those who make left turns (third exit) from the outside (right) lane are the reasons for most close calls, IMO. The next reason is those who don't yield to another vehicle in the roundabout.

And, yes, you can exit from the inside lane when making a left turn (third exit). There should not be another car beside you assuming no one has entered the roundabout improperly or is making an improper left turn from the right lane, since any car in the right lane entering the roundabout should either have taken the first exit (right turn) or second exit (straight ahead).

Use your signals. On my trips to the UK, it is common to use signals in roundabouts to let others know where you are going/exiting.