Ah, there've been other threads on this - trying to find them....

Have you had a transfer before? Some women find one or the other works better for them, though I'm not entirely sure why.
Overall, day 5 transfers are more successful, however, as mentioned, some women definitely find that day 3s work better for them.

At my clinic, it seems to correlate soemwhat to age (numbers below are just rough ideas, based on my memory or lack thereof).
For women aged <35, the success rate for a single blast transfer is about 50% vs 40% for single day 3 transfer
for women aged 35-39, the success rates are roughly the same, at around 35%
For women aged >39, the succes rate for a single blast transfer is about 5% vs 10% for a single day 3

I guess it also depends how many embryos you ahve at day 3 as to whether you want to chance trying for blasts (given that the attrition rate can be quite high).

We've done well with blastocyst transfers. But there are plenty of women who've had success with day 3s where they were unsuccessful with day 5s.

Sorry, I don't know that I was very helpful, but wishing you all the best