Aggregated demand of households given utility function
You just need to use the condition $$ MRS_{q_{1j},q_{2j}} = \frac{p_{1}}{p_{2}} $$ to obtain $$ 3 \frac{q_{2j}}{q_{1j}} = \frac{p_{1}}{p_{2}} \;\;\;\; \text{(1)}$$ Then solving for $p_{1}$ and plugging this into the budget constraint you obtain: $$ y = 3p_{2} \frac{q_{2j}}{q_{1j}}q_{1j} - p_{2}q_{2j}$$ $$ \Rightarrow q_{2j} = \frac{y/4}{p_{2}} $$ Accordingly, solving for $p_{2}$ from condition (1) and plugging this into the budget constraint yields: $$ q_{1j} = \frac{3y/4}{p_{1}} $$. 

From this you can obtain your function for the aggregated demand for goods $q_{1j},q_{2j}$, as a function of the prices.