For the reaction, $A+B\rightleftharpoons 3C$, if   '$a$' mol/litre of each 'A' and 'B' are taken initially then at equilibrium the incorrect relation is:

If a is the initial concentration & x is the amount A reacted, then at equilibrium,

$\begin{array}{c} \mathrm{A} + \mathrm{B} \rightleftharpoons 3\mathrm{C}\\ \mathrm{a} \mathrm{a} 0 \text{ (initial) }\\ \mathrm{a}-\mathrm{x} \mathrm{a}-\mathrm{x} 3\mathrm{x} \text{ (equilibrium) }\end{array}$

$\left[\mathrm{A}\right]+\left[\mathrm{B}\right]=\mathrm{a}-\mathrm{x}+\mathrm{a}-\mathrm{x}=2\mathrm{a}\equiv 3\left[\mathrm{C}\right]$

$\left[\mathrm{A}\right] + \left[\mathrm{B}\right] = 3\left[\mathrm{C}\right]$