In a large paella pan or a large sauté pan over medium-high heat, warm the olive oil. Add the sausage and cook, turning occasionally, until browned on both sides, about 3 minutes. Add the onion, bell pepper and garlic and cook, stirring occasionally, until softened, 3 to 4 minutes. Season with salt and pepper.
Add the rice, crumble in the saffron and cook, stirring, until the grains are well coated, about 2 minutes. Pour in the broth and stir in 1 1/2 tsp. salt. Bring to a boil, then reduce the heat to low, cover and cook until the rice has absorbed nearly all of the liquid, about 20 minutes.
While the rice is cooking, using a sharp paring knife or kitchen shears, cut along each shrimp’s shell above the vein line and then remove the vein. Leave the shell and tail intact.
When the rice has absorbed nearly all of the liquid, press the clams and mussels, hinge side down, into the rice, discarding any that do not close to the touch. Spread the shrimp over the rice and top with the peas. Cover and cook until the shrimp are opaque and the clams have opened, about 5 minutes. Discard any unopened clams and mussels and serve immediately.