Rules interps in an official event as a no - roger, I haven't done that here, just corrected a coupla misinterps by others

The only potentially iffy thing is the BW. I'd agree, for an official event, nerf it and don't hit the attended objects. There's enough UNattended objects that would clearly be hit to have fun enough with. Besides, these 3 green dragons, if properly played, could kill the party, twinks or no twinks, so go ahead and nerf that BW. It's still going to get all their pack saddles, saddles of non-mounted horses, and etc. Fyi - I'm the resident rules lawyer in our 3.5 DND game
Anyway - in researching those misinterps I discovered that the SRD actually has entries on the dragons

Never realized that (I play with the books 99% of the time

)
Now, you state that the players know the dragons are coming - I had though the dragons knew the players were coming, not vice verse. I'll assume both parties know the other is around.
That does not negate ambush tactics. Indeed, the green dragons, if they have any inkling of the formidability of the party, will seek to use ambush/guerilla tactics as much as possible. Heh, even Custer was known to say that the best way to spring an ambush was to walk into it...
That said, let me flesh out the dragons plan a little more and see what you think...
Sneed, Teed, and Meed are the dragons, let's say (or would you prefer Curly, Larry, and Moe?)
Sneed hides in a nearby lake/pond, with Teed and Meed covering the lake/pond in scum, etc, so that he can't be seen. Teed and Meed take cover within nearby forested areas. Using Summon Monster from sorceror spell list, bait and decoy the party to within 150' of Sneed. Dragon Fear hits, causing dispersion of riding animals, if any, or even party members with weak will saves. Teed and Meed take off, using the forest as cover, to setup strafing run. Party does whatever it wants to do - likely buff like mad, possibly attempt to detect the dragon. Misdirection and other Sorc spells will nicely contest any diviners in the group. Meanwhile, Teed and Meed load up on previously ripped up trees (they had warning, remember?) and take to high altitude bombing with huge clubs being thrown - follow the rules for thrown clubs, increase damage per the rules for weapon size. Compare thrown huge club damage + str to falling damage from same, and use the higher of the two (100% by the rules). Throw the clubs at around 200' away/up and you'll avoid 90% of their attacks at you, excepting only higher level/range spells, and well, you have a SR of 21, so that should hold you fairly well in combo with good saves.
While Teed and Meed are tossing logs at the invaders, Sneed is observing to pick out the cleric and the mage. Being huge, and allowed at least two attacks (one for each claw), his tactic after identifying the magic using targets will be to fly out from cover under the water, and using flyby attack and snatch, grapple the cleric and fly awy with him. Flyby attack allows the use of a standard action at any point during your move action. Thus, use a move action to double move, striking with a claw when passing the cleric. Start a grapple due to snatch and most likely win without some puissant buffs on that cleric. Finish your move action (300') by heading as up as you would allow as a GM.
Once Sneed makes his move, Teed and Meed hurl any remaining logs they are carrying and begin making low altitude strafing runs with their breath weapons. If necessary to prevent hostile missile fire, Sneed (before he makes his move) can cast darkness on the party's area, or if he's moved, either of the other dragons can once they start making their strafing runs.
If the magic users foil the dragons tactics before Sneed makes his move, Teed and Meed will retreat into the depths of the forest, and Sneed will lay low. These are all 5th level sorcerors and they'll know that effects like force wall, etc. will fade in minutes. They'll just retreat, lay low for an hour, and come back with more logs. If the party moves out from under protection, sneed can use message or whispering wind to communicate it to the others, and they can relaunch their attack very quickly.
All three are indeed 5th level sorcs, and thus have access to such things as true strike, bull strength, cats grace, fireball, acid arrow, etc.
All of which Sneed can toss from beneath his water covery without being detected - especially if such were coordinated with identical strikes from the others. With a little work, they could even present a convincing illusion that fireballs were coming 'through' the force wall that's put up. That might be a little iffy, but Sneed should be able to cast spells without hindrance from the water as the SRD indicates.
For brutal, straightforward tactics:
All thre manuever to opposite sides of the party without being seen. Via message they all launch an all out charge at the same time. All three moving at the same time, breathing at the same time, grappling three targets (one each) using snatch and crushing anyone not wielding a sharp weaon using the Crush feat. Round two - take off and leave anything not being held behind. Wait 15 minutes, buff, and repeat.
Simpler tactics, easier for a brute force minded party to deal with, could be more fun.