I started my older dog in obedience as something to do while waiting for her to be old enough for agility. I always thought it looked pretty boring but both the dog and I liked it. She broke her shoulder when she was 2 so just did obedience again until she was 3. Then most of the shows around where I live are joint shows...I went through a period where we just did agility... then did agility one day and obedience the other day. She is 5 1/2 yo and we recently started competing in tracking and have given up agility due to worrying about athritis etc in the shoulder.
She did pretty well in both sports... she wasn't more of an obedience or more of an agility dog. She was consistant in agility, got a lot of clear rounds and ribbons for most of those, but up in the top level she was always outclassed by the really fast dogs so did not make AgCh. She might get to ObCh yet. The higher we go in obedience the more she seems to enjoy the excercises.
Tracking in NZ is part of "working trials", about 2/3 of your marks are from the track, the rest heelwork, sendaway, scale jump, broad jump and retrieve over clear jump, plus 10 minute down stay. She loves tracking. I am actually very pleased she had previously done both obedience and agility as it made the rest of the working trials not too difficult, pretty much only had to learn the tracking.
My younger dog just does agility and she is faster and less obedient than Maddie, I have a lot of fun with her...she is trained to do obedience but doesn't compete yet, (dumbell issues). She was more attentive heeling at a younger age than Maddie but she doesn't have the same "style". I have also tried tracking with her but she does not show the same drive for it.
My next puppy I would hope to do something similar to Maddie (well without the broken shoulder...) do obedience, then agility, then both, then add tracking later on.