Hi Linda,
A bit of this depends on the stamina of your children, but if your kids are anything like mine were when they were younger, then I'm going to vote for the Pirates and Pals cruise. Here's why:
I love, love, love the Halloween party. The characters are wearing cool costumes, you get to trick or treat in the Magic Kingdom, there's a headless horseman at the start of the parade - all super fun. And your eight-year-old would have a grand time. However, the hours of the Halloween party are 7:00 p.m. until midnight. I can barely stay awake for the entire thing, I'd be surprised if toddlers could do this. When my kids were preschoolers, they usually went to bed around 7:30 or 8:00. I might be able to push them to stay awake a bit longer, but if I did, I'd pay the price the next day with cranky kiddos. Your two-year-old is free to enter the Halloween party, but my fear is that you'll pay the fee for the 3-year-old and he or she will fall asleep before even the first parade.
I also enjoy the Pirates and Pals cruise, but the shorter duration of this means that your kids might actually be awake for most of it. He focus here is comedy and characters. The cruise is lead by a humorous pirate named Patch (think puns, puns, puns) and ends with a private character greeting experience. Here too, I think the 8-year-old will get much more out of the experience than either of your younger kids.
To think outside the box for a second. Have you considered taking just your older child on one of these special excursions and leaving the younger ones with either your partner or a sitter. Disney contracts with a great sitting service,
Kid's Nite Out, who will send certified caregivers to your room. Many of the moms on this panel have used them with great success.
Good luck with your planning, please let us know if you need more input.