18 days past embryo transfer my hCG is 3665 iu/l what does this mean? (3 embryos transffered)

Well.... first, it certainly means you are pregnant. I suspect that number seems high and multiple pregnancies raise HCG, so, I suspect multiple pregnancies, while more than 3 is theoretically possible, 2 or 3 is what the result will be BUT, in any conception, the risk of very early embryo loss is quite high, so, successful pregnancy likely, multiple is likely and, you WILL need ultrasound in a few weeks.