Pupillages are, traditionally, broken down into first and second six. In the first six you do not have any rights of audience (except in certain tribunals - but we all have those already). You will be doing legal research, drafting opinions/advices for your supervisors, and generally getting to know your way around the system and your chambers.
In the second six you can start to develop a practice in your own right. You will, of course, be constantly supervised by a member of Chambers, and you won't be doing anything too complex, but the Clerks will start trying to find something for you to do - so you can earn some money.
In terms of cash, the first six will be paid via an award - a direct grant from chambers' funds. The second six will be a mixture of an award and guaranteed earnings. Anything above the guaranteed earnings is just your money (minus tax and clerks - some sets don't charge clerks fees during pupillage). Currently the minimum award is £1,000 a month. In the first six this is tax free. Depending on the practice area you may earn a lot more than that.
In terms of the third six, this is effectively a trial period at a new set. I don't know of anyone who has completed a full year pupillage with a set and then taken on a third six with them - normally if they keep you on, they give you tenancy (as I said, I know of no one, it doesn't mean it hasn't happened). A third six comes about if you don't get tenancy in your pupillage set, and you look to go elsewhere. The other set will not just take a punt on you, they will want time to evaluate you so they do another, shorter pupillage called the third six. I say shorter, but a friend of mine did a 9 months third six. Crazy. Unlike normal pupillage, the third six can turn into tenancy at any time - it's a good way to get your foot in the door of a new set.
Third sixes are funded differently from set to set, as far as I'm aware there's no funding requirement, some will fund you, some will give guaranteed earnings, some will just treat you like a tenant. Think of it as a probationary period.
Your earnings at the bar can not be precisely estimated - it depends on how good you are and how lucky you get. In terms of the areas you're looking at most criminal pupillages will pay the minimum £1,000. Family will pay slightly more than that, and property will pay slightly more again (all estimates, I don't know for certain). All I can say for certain is that, at the criminal bar, unless you are at a top, top set the firsy few years will be incredibly financially demanding.