Installing Debian packages from Stretch DVD 2 and 3 after installation using apt?

Run

apt-cdrom add

as root (or using sudo), and follow the prompts – it will ask you to insert a disk, then scan it and add the relevant information to /etc/apt/sources.list. You will then be able to install packages from it as usual.

In your /etc/apt/sources.list you will have an entry like this for the first DVD after installation:

deb cdrom:[Debian GNU/Linux 9.8.0 "Stretch" - Official amd64 DVD Binary-1 20190216-11:59]/ stretch contrib main

This line is used to tell apt to use the cdrom as a package source. The second and third DVD should be "Binary-2" and "Binary-3". The complete string can be found on the DVD in the file .disk/info.

If you only have the ISO files and no (virtual) optical drive is available, you can mount the images manually and add a deb file: source to /etc/apt/sources.list.

See How to use a .iso image as a CD-ROM Repository?