You obviously need enough (more than 6) to make blastocyst transfer worthwhile. I had twins via a transfer of 2 x day 3 frozen donor embryos, so day 3 is not intrinsically a worse option.
It is up to the embryologist and clinic to assess whether taking embryos to blastocyst is right for you. This will depend on how many embryos you have. If you only have 4 on day 3, it would be pretty easy to visually grad the 2/3 best for transfer. Taking to blast may risk those four and leave you with none to transfer. However, if you have 12 embies, then blast is a good way to weed out the front runners who make it to blast.
Also, the clinic need to be experts at blastocyst cultivation, otherwise they risk the embryos.
Basically day 3 or 5 transfers of donor eggs both result in high success rates. Don't forget these are young women's eggs, so day 3 is perfectly good for high chance of pregnancy IMHO.
What are your clinic's success rates for day 3 vs day 5 transfers with DE??
They can still only guess at actual chromosomal normality with both day 3 and 5 transfer, so the embies may look good but still be abnormal (or vice versa).