Center of Mass

First we calculate centre of mass of system let block A is at origin ( x=0 ) ‌xCM=‌

10(0)+25(10)+15(20)

10+25+15

‌xCM=‌

550

50

=11m
Now, new position of block A with respect to origin is x=2m and new position of block C,x=20−3=17m.
Let new position of block B with respect to origin is Y, so that centre of mass remains at same position
Then,
xCM‌=11=‌

10(2)+25(y)+15(17)

50

550‌=20+25y+255
25y‌=550−275=275
y‌=‌

275

25

=11m
So, block B should move 11−10=1 metre towards block C.