No Gen III games are needed; only Gen IV & V games are needed to obtain all Pokemon (legendaries and Kanto + Hoenn starters).
I'll list the hard(er) to obtain legendaries. You need:
Heart Gold & Soul Silver
- Both Lugia & Ho-oh are obtainable in just one of these editions
- Legendary Dogs (cats?)
- Weather Trio; Kyogre & Groudon are only available in one edition, and both are needed for Rayquaza
+ Kanto & Hoenn starters (one from each game, so you need to either trade or restart a game in order to get all 3).
- Giratina, Palkia & Dialga
Black & White
- Tornadus & Thundurus are version exclusive, both needed to obtain Landorus
Black 2 & White 2
- White Kyurem & Black Kyurem
Well, I guess you only need either Black 2 or White 2, since you can transfer either Zekrom or Reshiram from BW to fuse with Kyurem.
There are some legendaries available in multiple games, e.g. Lake Trio are available in Platinum & B2W2, and Sword trio are available in all Gen V games.
Note: you will either need to trade for some starters, or restart some games to get them.