Exam M is accepted by the SOA and CAS, while Course 3 is NOT accepted by the SOA. I would only recommend Course 3 if you are absolutely certain of pursuing the CAS. If you are unsure, take Exam M instead.
Here's the syllabus for Exam M.
Here's the syllabus for CAS Course 3.
I am planning on taking Exam M in November, 2006, and am using the ASM manual and Mahler's notes. Mahler's notes are intensive on CAS 3 concepts, but are missing Life Contingencies, which is over 50% of Exam M, so I'm also using ASM. Actuarial Mathematics is the syllabus recommended textbook, and used in NJIT's Math 441 and 442.
Exam M, for 'Modeling', is nicknamed the 'Mother of all exams', and is generally considered the weed out exam of aspiring actuaries, so be careful, as this test is rather difficult.
NOTE: The syllabus is changing. Here are updates for November, 2006, and the changes for 2007, when Loss Models material is moved to Exam C and Financial Economics is added to M, which will be increased from 4 to 5 hours long.
Syllabus Changes for November 2006 [pdf]
Syllabus Changes for 2007 [pdf]
Many students choose to go to seminars, but I have no experience with them.
Here are some study manuals:
Exam M:
Textbooks:
Introduction to Probability Models
Actexmadriver.com also has other study manuals for M and C, and solutions manuals for the textbooks.