First: I don't know for sure. Based upon some very limited testing & other research, I think OOTP just uses available stats and doesn't double up any years unless you select
double-weight. But your adjust/weaken settings can
very much come in to play.
So if you have three-year recalc enabled and 200 is your threshold for the
adjust setting, then you need 600 AB's over those three years (1987, 1988, 1989). If your rookie has 15 AB's in 1987 and 425 AB's in 1988, then he has 440 AB's. To get to 600, then 160 adjusted AB's will be added to his stats and then his ratings will be calculated based upon those 600 AB's. (In this example, if you use double-weight, then you are using those 15 AB's in '87 and 850 AB's in '88, and therefore no adjusted AB's.)
If, OTOH, your rookie had 15 AB's in 1987 and 575 AB's in 1988, then only 10 adjusted AB's will be applied, and he's basically going to be rated to perform like he did in 1988. (For double-weight, you'd have 15 AB's for '87 and 1150 AB's for '88, and no adjusted AB's...)
For a final-year player, his AB's for 1987 and 1988 will be used. If that total falls under 600 AB's, then adjusted AB's will apply. So for a guy who played fulltime in '87 & '88, he's surely going to be well over 600 AB's so his ratings will be based on his final two years... (and double-weight would do exactly what you would think: give double weight to his '88 stats compared to his '87 stats).
For your other examples, let's say a guy - for whatever reason - has only 275 total AB's for years 1987 thru 1989. Doesn't matter what year(s) those 275 AB's occurred, OOTP is going to add 325 adjusted AB's to his stats in order to come up with ratings. (For double-weight, he'd have 550 AB's and therefore only 50 adjusted AB's would be added.)
This is how I
think it works. I am not 100% certain on every detail, but I think it's at least pretty close
