If one creates an altcoin today, what are the 3 most important factors that would make it valuable? Or 3 activities that would do so?
You answered your own question:
People are what make a coin valuable.
A thing or service only has value , if another person is willing to trade it for something else.
Show a caveman some fiat cash , and unless he used it to wipe his butt, it would hold no other value for him.
But to a modern man , he could use it to buy necessities.
Show a modern man some flint rocks, and for him it has no value.
But to the cave man it would have more value than fiat, because the cave man could use it to make fire to cook his food or stay warm & dry.
All Value is subject to the use or trade that an individual sees for that object.
I see more value in ZEIT than I do in BTC, where as others see more value in another coin than BTC and others think BTC is all there is.
Just depends on an individual assessment for each coin.