5 minutes before serving, add peas, mussels and prawns, and squeeze 1 lemon over.
Stir again and leave to cook a further 5 minutes. Check seasoning. To serve, take the pan to the table so that guests can help themselves, and come back for more.
Remarks
To get closer to the "true" paella, I encourage you to use round Spanish rice rather than classic long grain.
Paella is one of those dishes that gets better each time you reheat it, so don't hesitate to make plenty at a time. It freezes very well.
It's important, in my opinion, that all the elements of a paella should be ready to eat, so no mussels, prawns or scampi still in their shells, or chicken with skin or bones. It takes longer, no doubt getting away from the true paella, but it's much more enjoyable to eat.