Let's denote the rate of work of person A, B, and C by a, b, and c, respectively. Then, we can write the following equations based on the information given:

1. A + B = 1/3 (they complete the work in 3 hours)
2. C + B = 1/4 (they complete the work in 4 hours)
3. A + C = 2/5 (they complete the work in 2.5 hours)

We can use these equations to find the individual rates of A, B, and C:

From equation 1, we have A = 1/3 - B From equation 2, we have C = 1/4 - B Substituting these into equation 3, we get: (1/3 - B) + (1/4 - B) = 2/5 Simplifying the above equation, we get B = 1/20

Therefore, A = 1/3 - 1/20 = 7/60 and C = 1/4 - 1/20 = 3/20

Now, the rate of work of all three working together is simply the sum of their individual rates: a + b + c = 7/60 + 1/20 + 3/20 = 1/10

Hence, it would take all three working together 10 hours to complete the job.