The rule is technically "3 years after HS graduation"
So, they can in theory declare for the draft after 3 years in the XFL.
XFL has said targeting pre-college players will be a focus of theirs. They're basically going to tell a dude from a poor background that they'll guarantee $1mn over 3 years, then they can declare for the NFL, or they can go to college and hope for the best. The strategy could be extremely successful.
They are equally competitors to college football as they are to the AAF / CFL.
PLAYER ELIGIBILITY
To be eligible for the draft, players must have been out of high school for at least three years
and must have used up their college eligibility before the start of the next college football season.
The NFL is very political. They'll make it difficult for player to skip college, go to the XFL and then to the NFL.
Sure, some players will take the instant money and go to the NFL but regardless of how much capital the XFL has backing it, they have to make a profit. It will be difficult for the XFL to pay NFL type contracts which would keep star players from considering it.
It appears the AAF has locked players into 3 year non-guaranteed contracts. That takes a lot of the easily acquired players off the market for the XFL.
They can't depend on very many players that didn't go to college. That would look silly compared to the AAF players many of whom have played some snaps in the NFL and most have college starting experience.
The XFL can't recruit college players directly. If they screw up a few kids college eligibility, the Govt will shut them down.
If they manage to lure some star college recruits away from college, all of the congress members in that state will be heros for going after the XFL.
Obviously it's not impossible for the XFL to succeed but the challenges are immense.