There are 2 problems. First, as you found, Dell is not going to guarantee that the Win 10 drivers will work on Win 7. The drivers might work work or you might get unexplained glitches and system stability issues. Second problem is how to install Win 7. Win 7 media doesn't include the USB drivers that are compatible for the install. You need to create a bootable thumb drive with the Win 7 iso file and slipstream in the correct USB drivers. Overall, a lot of work and potential problems to install Win 7.
What's wrong with using Win 10? The Dell 3560 is built to run correctly with Win 10.
I am going to have my son try the things that might get me win 7. I am 89 years old, and not to smart on pc's
I will get back after the weekend and let you know what happened.
Thanks a million. I have been trying for 2 years to get it to work with no luck.