I know this post is a month old so you may have started taking the tests already, but I've just completed the last of the three upgrade exams, so thought I'd chip in.
As you've said, the three exams you need are 70-457, 70-458 and 70-459. Of the three, I found 70-458 by far the most challenging as it required some quite in-depth studying of SSIS which I had relatively little experience with. The other two were ok, and didn't require much as much study.
You can get most of the information from the Microsoft Learning website, but essentially the exams cover:
70-457 - Admin, Querying
70-458 - Admin, SSIS/Master Data Services/Data Quality Services/Data Warehousing
70-459 - Admin, Development
I used the three Microsoft MCSA books for studying, and went through all the examples for the "Implementing a Data Warehouse" book. For the other two books I read through most of the Querying book (mostly because I found it quite an enjoyable read!) and skim-read the Admin book, focussing on the bits that were new to SQL 2012 - e.g. AlwaysOn. According to the Microsoft Learning website
, most of the 70-459 exam content seemed to be covered by the Querying and Admin books, so I just used those plus Books Online for any bits that weren't covered. In any case, if you read the Skills Measured you'll see that exam is more about the application of knowledge than the knowledge itself, so that's where on-the-job experience and general reading around the subject will help enormously.
I must say, I've enjoyed the whole process of studying for these exams, and learned quite a few new things along the way - which is why I force myself to go through this process every few years (roll on SQL 2014!). I thought the study books were all well-written and the exams were very fair in the sorts of questions they asked.
So good luck, and I hope you do well. Any help I can give you without breaking NDAs, I'm happy to give, so ask if you've got any more questions.