ADD: This one probably has more potential for trouble than the others. Look in the Pilot Health forum, there are several discussions about ADD. You will probably have a battle to fight with the FAA, but once you prevail you should have clear sailing ahead, since ADD is primarily an issue with kids and teenagers.

Depression: This is common in the population as a whole, so it is common among pilots as well. The key is that you cannot hold a medical while taking any medications, and there is a waiting period after you finish meds (used to be six months, not sure what it is today). You must also be able to function normally without the meds...basically a past history of depression is not an issue with the FAA, as long as you do not have an ongoing problem (sounds like you don't). You will have to provide a letter from your mental health care provider, talk to the AME for details.

Marijuana: Since you got treatment, I think you will have to report it to the FAA (read the medical application form online). The good news is that nobody really thinks pot is particularly addictive, so as long as you are drug free now it should not be a medical certification problem. Since no law enforcement was ever involved, it should not be an issue for employers (you probably don't even have to tell them).

The pot and depression issues sound straightforward, I think you could work with the AME directly on those. But you may want to talk to an aviation medical consultant about the ADD before you talk to the AME...there may be some pitfalls with that. I have used this service, and there are others out there:

http://www.aviationmedicine.com/

Also be aware that in the past it was common for major airlines to perform extensive medical exams on applicants, with standards far higher than the FAA. The good news is that many major airlines (and all regionals) will now accept just an a first class FAA medical, and will not get more invasive than that. But there may be a few majors which would still reject you, but there's no way to know for sure until you get to that point.

Good Luck