It's important to specify at which frequency you want unity gain. But assuming you mean DC ($\omega=0$) because that filter has a low pass characteristic, the DC gain of an IIR filter is given by
$$G_{DC}=\frac{\sum_kb[k]}{\sum_ka[k]}\tag{1}$$
It's common to normalize the denominator coefficients such that $a[0]=1$. In your example that would give
a = [1.00000 -0.29240 0.10849]
and
b = [0 1209000000]