if the average of A and B is 2.5, then

(A + B)/2 = 2.5

=> A + B = 5

if the average of B and C is 3.8, then

(B + C)/2 = 3.8

=> B + C = 7.6

if the average of A and C is 3.1, then

(A + C)/2 = 3.1

=> A + C = 6.2

thus we have three simultaneous equations:

A + B = 5 ......................(1)

B + C = 7.6 ....................(2)

A + C = 6.2 ....................(3)

subtract eqaution (1) from (3), we get:

C - B = 1.2 ..............(4) = (3) - (1)

B + C = 7.6 ...............(2)

add these two equations, we get:

2C = 8.8

so C = 4.4

but A + C = 6.2 ...............(3)

so A + 4.4 = 6.2

so A = 6.2 - 4.4 = 1.8

but A + B = 5 ..................(1)

so 1.8 + B = 5

so B = 5 - 1.8 = 3.2

so we have

A = 1.8, B = 3.2, and C = 4.4