How to hang train on the hanger

I'm not sure what you mean by three straps. Most just have one strap. Could you take a picture?

It is probably better just to have a steamer available as needed -- and if there is no warning against steaming your dress. The other option mentioned is to hang the dress in the bathroom where the steam of the shower will help remove wrinkles.

Are there any hidden loops in one of the top layers that you're supposed to feed the straps through? If not, then I think you need clothespins and maybe a second hanger to hang the train on. I would hang the straps on the hanger as they're arranged - one on the left, one around the hanger hook, and one on the right - and then use clothespins to hang up the other layers of the train so that there's minimal bunching. If what I'm envisioning is accurate, the other layers will still be on the inside of the layer with the straps when you hang it, but the point is to protect the dress, not really show it off. As someone has already mentioned, you'll still need a steamer.

Side note: I'm not really surprised that the straps are halfway up the train on an inner layer. Since it's so long, it distributes the weight better to put them in the middle than putting them at the end. You also don't want gravity to do anything weird to the top layer, which is why the straps are on the inside that will be hidden. And now that I'm thinking of it, the weight of the train will be on the inner layer if you clothespin the other layers to it.

Anyway, I hope this is helpful! Sometimes what I see in my head does not turn out to be how it works in reality

Can you check with the boutique or designer of the dress? Maybe another boutique could answer the question as well.