Bring a large pot of water to a boil for the pasta.
Meanwhile, heat a large, deep skillet over medium-high heat. Add the Italian sausage and cook, breaking up the meat with the edge of a spoon, until cooked through and no longer pink, about 5 minutes. Using a slotted spoon, transfer the cooked sausage to a plate and set aside.
If there is excess fat in the skillet, discard all but about 1 tablespoon of the fat. If the skillet seems dry, add 1 tablespoon olive oil.
Add the shrimp and the halibut pieces to the skillet in a single layer. Season with the salt and pepper and cook until just cooked through, 2 to 3 minutes per side. Transfer cooked fish to a plate and set aside.
Reduce the heat to medium and add the shallot, garlic, and red pepper flakes and cook, stirring, until the shallots are tender, about 2 minutes.
Add the anchovies and cook, breaking up the anchovies with the edge of a spoon so that they melt and turn into a paste.
Add the cherry tomatoes and 1/4 cup of the white wine and stir, scraping up any browned bits on the bottom of the pan. Cook, tossing, so that they begin to blister and burst open, about 5 minutes. Using a potato masher or the edge of a spoon, press on the tomatoes so they pop open and the juices release from the tomatoes.
Add the tarragon, parsley and pour in the remaining 3/4 cup of white wine. Bring to a simmer and cook, simmering lightly, so that it reduces and the flavors meld, about 4 minutes. If it starts to reduce too much cover and keep on low while the pasta cooks.
Meanwhile, when your water is boiling add the pasta to the water along with plenty of salt (about a tablespoon). Boil until al dente, according to package directions.
Transfer the cooked pasta to the sauce and add the canned clams (undrained) and the cooked sausage. Toss until the pasta is well coated in the sauce and glossy, 2 to 3 minutes.
Remove from the heat and add the cooked shrimp and halibut to the pasta and gently toss once more so that it well combined. Taste and add more salt, if desired.
Serve and garnish with more freshly chopped parsley and tarragon.