well, from what i've read they initially had those seperate teams for ep1 and ep2 who started at the same time, and once ep1 was released that team started to work on ep3.
but when they decided to put 3 new games into the orange box, they reassigned everybody from the ep3 team to the tf2 and portal teams to get it done (nearly) in time.
after the orange box was released, i guess some of the guys went back to ep3 already.
but then they bought turtle rocks studios and concentrated most of their efforts on finishing l4d. since they had a lot of guys still working on the tf2 updates, i guess there wheren't too many left to work on ep3 at that time.
now they still have to maintain tf2 and l4d with updates, but most of the devs should be working on new projects now. that's ep3 and portal 2.
so all in all, they should have the basic concepts layed out already, but i seriously doubt it that they are very far into the developement right now. it will take at least 2 more years before we can say anything more certain.
I believe if it would take another 2 years, it would be more half-life 3 then hl2 ep3. From what i gather from valve's info on this in emails, interviews etc, ep 3 is pretty far on the way, not just the basics and concepts.