Let the 3 forces be F, G and H.

We are told F + G = H .....(1)

Also, the second force, G = F + 6.4 .....(2)

For the third force, H = 4 × F .....(3)

If we substitute equations (2) and (3) into (1), we have:

So F = 3.2

This gives G = 9.6 and H = 12.8.

Checking:

(1) F + G = 3.2 + 9.6 = 12.8 = H (OK)

(2) F + 6.4 = 3.2 + 6.4 = 9.6 (OK)

(3) 4F = 4 × 3.2 = 12.8 (OK)

So the three forces are 3.2 N, 9.6 N and 12.8 N, respectively.