How do you do the look with the one strap around your neck like the first picture listed?

The strap around the neck belongs to the Bra. This low back bra converter designed for lower back or backless clothing like evening dress, convert any regular bra into a lower deep V shaped back bra, give you a smooth & natural no show look.

what is the difference between the 3 hook and 2 hook?

The 3 hook one is 3 hook 3 row, 3/4 inch hook center spacing, you can use it on 3 hooks bras with the same hook center spacing. The 2 hook one is 2 hook, 3 row, 3/4 inch hook center spacing, you can use it on 2 hook bras with the same hook center spacing.

You can follow the below instruction to put the straps on. I have uploaded a picture on the listing, it shows how to put the straps on. You may check it. 1. Attach the straps to your bra's hook parts. 2. criss-cross the straps on the back 3. fastern the straps in front. It is done.

Can this be used with a strapless bra?

I wouldnt think so, when using these straps much of the support is transferred from the band to the shoulder straps. The bra would get pulled down with a strapless bra.