One obvious tack, after a rotation, is to look at cards that will be rotating in that may fill the hole left by the cards rotating out. In the 3cc range for a mono green deck, this card immediately caught my eye:
Trample. Splinterfright's power and toughness are each equal to the
number of creature cards in your graveyard. At the beginning of your
upkeep, put the top two cards of your library into your graveyard.
Alternatively, this doesn't look too bad in a green deck packing a lot of creatures, though it would probably work better with lots of tramplers (4 Garruk's Companions would be a good start):
Elder of Laurels (2G)
2/3. 3G: Target creature gets +X/+X until end of turn, where X is the
number of creatures you control.
Not in the 3cc slot, sadly, but this card looks like the poster child for mono green horde decks in the new set:
Essence of the Wild (3GGG):
6/6. Creatures you control enter the battlefield as a copy of Essence
of the Wild.
Unfortunately all of these are rares or mythics, but it's that or go werewolves basically...