I've been on the hunt for G3's for 3 months now and I've noticed some ponies that seem rather hard to find (at least for me). So I can only give you a list of those.
Beach Belle
Carribean Delight
Crowning Glory
Pretty Palace
Royal Beauty
Aurora Mist
Pearly Pie
Daisy Paisley
Winter Miny - Year 6
Egg Pony Sweetie Belle - Year 6
Egg Pony Pinkie Pie - Year 6
I've see quite alot of the two faced comic con pony and wouldn't call that hard to find, yet the prices are abit high. Same with the black comic con super hero pony. It's not a lack of them that makes it difficult for me to obtain them, it's their prices
Other than that there's the obvious ones:
Rosey Posey
Bay Breeze
The Pony Project Ponies (I've been looking for the black one without any luck for 3 months O.o, the others do pop up every now and then)
Twinkle Hope
Breast Cancer Pony
That's all the help I can be I'm afraid. And btw, if you're looking to sell any of those let me know
I'd be more than interested hehe.