You need to trust your doctors and the embryologist. If the embryos are only 4-6 cells on day three, that means that they are already not growing at the expected rate (should be 8 cells) and by attempting to take them all to blast, you risk having nothing at all to transfer.
The problem is that this is still an imperfect process. The embryologists are making educated guesses about which embryos are the most likely to keep growing, implant into the uterine wall and continue to divide and produce a healthy pregnancy-there is no tool that is going to tell them this information.
Despite the premise that embryos transferred at the blast stage may be a little more likely to keep going, there are lots and lots of babies which came from bad-looking and less than optimal embryos. (Our ds is from a three-day transfer of one 6 cell and two 5 cell---many years ago, mind you!)
The current rate of growth is not optimum--if you transfer the best of the bunch now at three days, you may hit the right embryo. But it is highly unlikely that these will make it to blastocysts without arresting.
Listen to your RE and the embryologist.