He has to make a bowl next year. Not saying he wouldn't be back for year four even minus a bowl game in his first three years (and 4 straight overall for Nebraska), but he'd definitely be on the hot seat in year four.
As you are suggesting, I don't think five years is an absolute given. As I've said before, Frost is getting more rope and forgiveness simply because of his name (any other coach would be on a hot seat much sooner). But a ton of shine has worn off these past two losses (Indiana, Purdue). I've seen a lot of people that were still quite confident not more than 2-3 weeks ago now expressing significant concerns. If we don't make a bowl game in 2020, the vast majority of the Nebraska fan base will be very displeased. And then I would start to worry about ticket sales in 2021. I'm not worried about sales for next season -- but one more year similar to this, and I'd be concerned. Tough decisions would need to be made if selling tickets ever became an issue.