BIAB and 3 vessel brewing are just two different ways of doing it.
I saw the three vessel way of doing it in London and it's very easy to follow, methodical but you've got two lots of cleaning to do which isn't much of a hardship.
Doing BIAB you've only got the one pot to clean and a bag, so it's similar. BIAB is also less efficient so you'll need to add extra grain.
I've gone down BIAB and there's a good community with a good spreadsheet for working out your brews on
www.biabrewer.info. You can download sample recipes and just follow the simple steps.
You also have the benefit of doing maxi-biab which means if you have trouble getting a large enough pot, you can add extra water throughout the process to make up volume.
So in short, BIAB is a little more expensive in ingredients, cheaper on initial starting equipment and takes up a little less space. I think it's a winner personally.
The cheapest way to start with BIAB is to get a cheap fermentation bucket such as you'd find at wilkos (£6.80 currently) and a couple of tesco value kettles (about £6 each)
viewtopic.php?f=30&t=36368
Then you just need a bit of voille (think net curtain) material for the grain sack and a thermometer.
This does require some electrical knowledge, also you need to make sure you don't plug into the same plug socket or ring main (note: you need two separate ring mains).
Alternatively get a stainless steel pot and a big burner, or make an investment and build a full on electric brewery!
http://www.theelectricbrewery.com/ (I built a similar one pot system with a much smaller panel)
Have fun