Let A, B, C represent their respective weights. Then, we have:
A + B + C = (45 x 3) = 135 .... (i)
A + B = (40 x 2) = 80 .... (ii)
B + C = (43 x 2) = 86 ....(iii)
Adding (ii) and (iii), we get, A + 2B + C = 166 .... (iv)
Subtracting (i) from (iv); we get, B = 31.
Therefore, B's weight = 31 kg.