When a young man comes across a computer pop-up ad that informs him he will be killed in 3 days, he must do what he can to save himself.

False premise. I don't believe a person would see a pop up add saying you're going to die in 3 days and go oh snap, I didn't know that, let me see what I can do about it! And anyone who would react that way is not someone I want to watch make other decisions for 90 minutes.

I would slightly change the concept from Internet pop up to funny app that has been downloaded.

When users of an app that predicts your time of death starts dying at exactly the predicted time, a young Hacker must track down its origin to save himself.

Like said by others, a very visual cue, like a giant countdown pop up that appears whenever he goes: digital billboard, airport airport/departure board, Internet screen, phone, TV screen.

It's a good concept for a mix of?The Ring, Final Destination?and D.O.A.

To add tension, depending of what the person does, the countdown can jump speed down. People can buy time by doing nasty thing like in GTA. Push a grand-mother under a bus, increase your immediate countdown but increase your chance of being killed later.

I would suggest the title:?Game of Life.

"...he must do what he can to save himself" is redundant. Edit instead with an objective-visual-goal such as "....he must track down the creator of that O.S. to save himself"
Good Luck!!