Stackleburg Oligopoly 3 firms

magine that there are 3 firms in a monopolistic market, F1, F2 and F3. Firms 1 and 2 are incumbent firms and act simultaneously whereas Firm 3 observes the actions of both firms before deciding whether to enter.

All firms face a market price of 𝑃(𝑄)=16−𝑞1−𝑞2−𝑞3. Incumbent firms face a production cost of 𝐶𝑖(𝑞𝑖)=4𝑞𝑖 for 𝑖=1,2. Firm 3 faces a production cost of 𝐶3(𝑞3)=4+4𝑞3. Find both firm 3 and firm 1's payoff.

I understand that I need to take firm 3's best response function which is
0.5(12−𝑞2−𝑞3)
but I am not sure where to go from there. How would I go about finding each firms predicted quantity? Thanks!