Congratulations on the three little angels! Happy to hear they're all safe and sound.
. As for telling them, there are a bunch of little kid books on Amazon that gently explain it. I bought about 7 of them and they're all great. They all seem to be for slightly different ages. Some are somewhat graphic and actually explain sperm, etc so my husband and I will read that one when they're a bit older. Meanwhile, we incorporate the most gentle books in our bedtime sleep routine to our 5 month old twins. (We read to them every night)
We want it to be a familiar story so it's easier to talk about when they're ready to understand.
Until then, enjoy!