(2) From the meaning of the problem:
400 x+550y+500(500 x-y)= 235000
Finishing: y = 2x-300；;
(3)①P =(500-400)x+(800-550)y+(650-500)(500-x-y)-2000，
And y = 2x-300,
∴ Sorting: P= 150x+43000
② The quantity of C brand down jackets purchased is:
500 x-y = 500 x-(2x-300)= 800-3x
According to the meaning of the problem, list the inequality groups.
x≥ 1002x？ 300≥ 100800? 3x≥ 100，
Solution: 200≤x≤7003
The range of ∴x is 200≤x≤x7003,
X is an integer, and the maximum value of X is 233.
∫ in P= 150x+43000, k = 150 > 0,
∴P increases with the increase of X.
When the maximum value of x is 233, the maximum value of p is 77950 yuan.
At this time, 233 sets of A, B and C brand down jackets were purchased, which were 166 sets and10/set respectively.