I have several sizes listed for Asha OECs:http://betterthandiamond.com/discussion/ubbthreads.php/topics/739067/1
I'm not sure how helpful this will be considering all the sizes I purchased were from 6.5 -8mm.
But based on a 7.5 ring size, I'd say a 3ct, won't look too big at all.
I'm a size 5.5 and the 8mm stone was the perfect size for someone worried about "shrinkage" on a 5.5. I personally wouldn't go over an 8mm for 5.5, but I'm a weirdo who likes small.
But for a size 7.5, I'd say go for the 3ct. I think it may be just right for you.