Only the DVD-1 file is required for installation. The DVD-2 and DVD-3 files contain additional packages that are only optional.
From Debian's website:
There are lots of files here! Do I need all of them?
In most cases it is not necessary to download and use all of these
images to be able to install Debian on your computer. Debian comes
with a massive set of software packages, hence why it takes so many
disks for a complete set. Most typical users only need a small subset
of those software packages.
Initially, you will only need to download and use the first image of a
set (labelled as debian-something-1 to be able to start the Debian
installer and set up Debian on your computer. If there are more images
available here (labelled debian-something-2, debian-something-3,
etc.), they contain the extra packages that can be installed on a
Debian system (as mentioned previously). They will not be bootable and
are entirely optional. If you have a fast Internet connection, you're
most likely better off installing any desired extra packages directly
from the Debian mirrors on the Internet instead of by using these
The ISO file size for DVD-1 is listed as 3.7GB. The link for the torrent download is http://cdimage.debian.org/debian-cd/8.6.0/amd64/bt-dvd/debian-8.6.0-amd64-DVD-1.iso.torrent