iTunes used to have a Downloads window. Then it disappeared. But some people, including me, see that window, even though I can’t figure out how it displays.
But iTunes 12.3 now has an Activity Window, which is, for all intents and purposes, the same as the previous Downloads window (with a slightly different display). To view this window, choose Window > Activity Window. When you start a download, you’ll see it in the window. And you have the options to choose Simultaneous Download and to pause downloads (which you also see when clicking the status circle at the right of the iTunes toolbar).
If you missed the old Downloads window, use the new Activity window to keep an eye on your downloads.