The story with three dead Supermen was in Action Comics #399, April 1971, "Superman, You're Dead ... Dead ... Dead!". It didn't involve aliens exactly but the 24th century Historical Foundation of Earth which used a chrono-selector to snatch various heroes from the past just before they died so history students could study them first-hand. It turned out that they were from an alternate universe which unintentionally tore a hole into Superman's mainstream Earth and time. It was reprinted in the hardcover Superman From the 30's to the 70's. The hardcover you're seeking may be this book.
I'd need some more details on the tumours story. It sounds like one of the classic Virux X tales when Superman was really dying. If it's the Virus X story you meant and I'm thinking of, it turned out Virus X is plant-like enough to be destroyed by white kryptonite which the Bizarros had strewn over Superman's funeral coffin as a 'gift' to help him die faster.
Thanks for the info. That does sound like the issue I was talking about. Also, I believe the book I saw was Superman From The 30's To The 80's. I can't find a picture of the cover anywhere, but I'm almost certain that's it. It had some parts from the comic I had mentioned in it.
It also featured the cover of an issue of Superman that I've been unable to locate. On the front, someone is shooting Superman with some age-accelerating ray. Superman is old looking, reaching up to the guy while the guy says "By the time you reach me Superman, you'll be dead!" Does anyone know what issue this is?