Is allowed using of UCS ?
If so, then it's simple :
1. pick pt1 and pt2 in current UCS
2. calculate (setq mid (mapcar '(lambda ( a b ) (/ (+ a b) 2.0)) pt1 pt2))
3. pick pt3
4. (setq pt3w (trans pt3 1 0))
5. (command "_.UCS" "_3P" "_non" mid "_non" pt2 "")
6. (setq pt3n (trans pt3w 0 1))
7. (cond
( (and (> (car pt3n) 0) (> (cadr pt3n) 0))
(prompt "\nQuadrant A...")
)
( (and (< (car pt3n) 0) (> (cadr pt3n) 0))
(prompt "\nQuadrant B...")
)
( (and (< (car pt3n) 0) (< (cadr pt3n) 0))
(prompt "\nQuadrant C...")
)
( (and (> (car pt3n) 0) (< (cadr pt3n) 0))
(prompt "\nQuadrant D...")
)
)
......
HTH.