On day 3 they expect them too be about 8-cell to receive a high rating. I had on day 3: two 5-cell and one 6-cell; I have a girl now. There are many stories of 4-6 cell embryos (below 8 cell that is expecting) implanting and resulting in pregnancy.
I am so very sorry this is so stressful. Typically RE's would cancel a cycle and try a different protocol with a poor responder (less than 4 follicles). I am so sorry you are going through this.
My clinic does 3 day transfers so today is 2 day tomorrow is my transfer the embryoligest didnt seem concerned she said it was growing a lil slow but normal
Most fertility clinics these days prefer to transfer 5 day blasts as opposed to 3 day old embies. By the third day I think they expect them to be at least 5 cells and by day 5 they expect them to be over 100 cells. My embryos stopped dividing at day 2 and reached 4 cells only. I had nothing to transfer at day 5. I'm not sure what is going to happen with yours but I would be prepared for bad news, I'm sorry. Did your RE use the most aggressive drugs for you?