ooo This'll be fun
I'll assume the following points (correct me if I'm wrong on any, but the general concepts should still apply barring major inaccuracies) since I don't have my books on me at the moment.
1) 3 Adult Green Dragons = EL ~14-15, thus your party's avg lvl is 13-15?
2) Adult Green Dragon BW = 10D6 Cone Acid Gas? With full corrosive capabilities? 1/1d4 rnds, right?
Terrain = forest edge and they have been warned (always a nasty thing)
First things first - remember that dragons are characters too. Be sure to fluff out their full skills and feats that are possible to them. Consider dropping to young adult greens in order to add in class levels and really shock your party
Forget about starting with buff spells

They tend to fade fast in a guerilla style of warfare tactics - and what sane dragon is going to go toe to toe with a party without doing something to soften them up first? Well, maybe a young dragon would, but soon enough they'll learn that arrows, fireballs, magic missiles, etc. HURT if you get too close

. Traps in the woods would be great fun to lead party members into if chased.
Ambush to start - as the party is trooping along (floating, walking, riding, etc.) the first green dragon makes a full move action at the max height he can fly and still use his BW on the party, from behind the party. Give 'em a DC25 spot check or spot check opposed to the dragons move silently rolls. Even if they spot the dragon, there's no time to react before the cone of acid gas + dragon fear washes over the party. Remember to have each riding animal make it's saving throw vs dragon fear (will save) or gallop away out of control - this could seperate your party, so be prepared in order to avoid a TPK. Also don't forget the affect of an acid cloud on party equipment... mage robes, swords, etc. - that'll apply to anyone who actually failed their reflex saving throws, i.e. hopefully the fighters
The party will start reacting, but the dragon just keeps flying away. If they are all focused on the first one, the second makes the same attack run if no magic has been unleashed (and be wary of readied actions and requisite specificity of same - if they trigger off the first dragon, they don't trigger off the second

) If the party has split, the second one can veer after any obvious fighter character types that are split off and do a snatch and fling, or just snatch and fly high to drop or snatch, fly high and throw on second init.
By this time, the party will gear up for a dragon flight. Flight spells, walls of force, etc. That's fine. The dragons leave, maybe they got one (the one that split off) or maybe not. 40' flight will outrun a wizards flight nicely. The third dragon remains well hidden, under water or wherever, and observes the party, identifying mages, clerics, and fighter types.
Peace descends upon the party for an hour or two, until they start to move again. More braced this time, they'll observe the two dragons apparently returning - merely flying much higher. Make projectile rolls at 250' up in the air for the dragons dropping medium load boulders at the party, and let the dragons fly off. If the party is ready, this'll absorb more force walls, healings, etc. If not, this'll hurt a lot and then absorb quite a few healings - assuming the dragons come close. Whether or not they do, every fifteen minutes or however long you deem it would take to retrieve trees or boulders, new stuff drops from the sky on them.
Sooner or later, they'll retreat into the forest to spoil the aerial aim. There's not really a whole lot else they can do against this kind of attack especially once the mage starts running low on spells. At this point, switch from high altitude bombing to low altitude, high speed strafing runs at intermittent periods with BW's active.
That's how I'd at least open up. I'll post back later tonight when I have my books and have thought about this some more. I'm thinking they should have shapeshifting of some form and can turn humanoid? If so, the third dragon can skulk in through the woods fairly close, shift, and do a snatch/fly to leave with a cleric/healer type. There's more and other stuff they could do tactically, just depending on exactly what they can do... Can't they do a chlorine gas attack as well? Just find a hollow spot the party will walk through and fill it with chlorine, then BW attack them while they are in it. With a few spells, magic items of their own, they could even fence the party in and let 'em suffocate
But more precise stuff later.
Do the greens have magic items? I'd think it'd be unfair, and you'd be leaning towards a TPK... but, ya know.
Any help so far?