The folks above are right, your overall sales for a sequel will usually be lower than the first book in the series, just because not everyone who buys the first one will want to keep going. That said, ideally you want the size of your launch sales to grow, since it means your overall base of readers is expanding with each installment. But that takes time, and getting the word out about launches is half the battle, so don't worry if you don't see that growth on book #3.
As for marketing, it's hard to give guidance on that. All of the truly reliable tools like Bookbub have became so popular that you can't count on being able to use them. A Bookbub sale is always nice, but unless you have a Big publisher in your pocket, you shouldn't count on being able to get one. On the other hand, there are countless imitators that you can use. Success with those is a crapshoot, of course, otherwise they'd already be booked up, but there's nothing inherently wrong with playing craps if you can afford it.
My advice would be to use your biggest advantage: the serial itself. Having a direct line to your readers is something loads of authors would love. Make sure they know about the launch, and that early support is a big help. If it's possible to incentivize awareness or participation, all the better. Personally, I like doing big Digital Launch Parties where I have games, prizes, guests, and stuff like that to celebrate a launch. Fun for me, fun for the readers, and it gives them a reason to care about launch dates, which in turn helps early sales and pushes up the rankings. That's just me and my brand though, any method that engages the readers would work.