i would take at least a 4th walkon before signing a random unscouted player.
scratch that, i forgot they force you to take dumb players instead of walkons now. i would by a huge margin take that 3rd walkon before signing some random dude. but may as well sign a random dude instead of the almost guaranteed terrible player they will sign for you for that 4th walkon. doesn't matter a ton, either way you are probably cutting them for next season. but at least with a random dude, its only perhaps 90% you cut them, as opposed to say 99% with the guy the sim forces you to sign.
we generally call everything but FT% from high school stats (if it even still shows there) window dressing because the information you get from ratings is infinitely more accurate, and is complete on a relative basis (in the presence of known ratings, the high school stats provide zero additional value). however, in absence of ratings, i wouldn't call high school stats 100% irrelevant. this is a pretty obscure edge case, almost never would you want to evaluate off high school stats in lieu of taking a walkon. however, if i was a betting man, i would take the guy with better high school stats.
however, note that for sure, those high school stats DO NOT include potential / maximum ratings. if they are correlated to anything (which i think some are), its starting ratings only. so we are talking very fringe value here. still, if you are tossing a coin based on nothing otherwise - i'd probably use them? however, i do think what other folks said, finding players that had some interest, etc, are more useful than the high school stats. i didn't think you could see the considering list or preferences (almost definitely not preferences? scouting level 2?) without having scouted the guy somewhat though. so i'm not entirely convinced any of that is a real option for you (except the transfer comment - that is legit - you should be able to find the ratings for all transfers as long as you can find their prior team), but the finer points of 3.0 recruiting are beyond my range of intimate understanding so i'm really not sure.