Read a lot about 3 Act, 5 Act (The Hulk, great article) and 8 Sequence.
What do you use and why?
I’m writing my novel using the 8 Sequence Structure (within the 3 Act), but I struggle with the second Sequence, Predicament and Lock-In. Maybe I’ll have to push a fe scenes around. The other sequences fit well, at least in the first draft.
My advice would be - don’t struggle! For decades, if not centuries, people successfully wrote novels without caring too much about the details of acts, sequences or, indeed, structures. They simply wrote stories that they believed would be interesting to readers.
Maintaining interest is the key; structures are just one means to that end. Read a lot - and I mean a lot - in the genre(s) you intend to write in, note carefully what seems to work, and more importantly, for you as a reader, why, and then - write*.
If anything, leave the structure-fying till afterwards - as a check during revision that you haven’t missed a trick of plot or character along the way. But don’t let someone else’s structures rule. This is your story, your rules.
My (unstructured) 2c.
*By which I do not mean, don’t outline.
My thoughts on structures:
1) All the different structures (you mention 3 act, 5 part and 8 sequence, but could just as easily add the Save The Cat 15 beats) are exactly the same thing, just with more detail.
Extrapolate and go for the 40 chapter structure from the nearest book on your shelf and it becomes obvious that the more rigidly you follow any of them the more formulaic you become. My advice is to read them all. Understand why the structure has been articulated the way it has and then kind of just forget about them while planning your own story.
2) They all are just stating what is obvious to us all as readers.
So in the 3 act version: Stories have a Beginning, a Middle, and an end. Obvious, huh?
Longer structures have a period of ‘lock-in’ at the end of the first act? This is just another way of saying you must have some reason why your character continues to put himself in harms way rather than just quitting and doing something else. It makes sense for this to happen early (around the end of the first act) simply because there needs to be something to be locked into first, but it needs to happen early enough that the character doesn’t start having nasty things that would make a normal person (and your reader) walk away.
Is a lock-in moment essential? No. We understand that a murder detective in a murder detective novel isn’t walking away because it’s his job. But in the absence of such natural ties it’s sometimes helpful to give the protagonist a reason to continue hanging around the world you have created.
Not rocket science. If you understand the story you are telling and motivations of your characters, all those beats / sequences / parts should be things that naturally fall into place as and when you need them.
Thanks Hugh and Pigfender for your advices. They are much appreciated.
Being a real newbie with novel writing my biggest fear was to bla around without any heading or structure. I started off with an idea and then wrote a treatment AND even a logline, yes I did 
(t’was hard, I must admit, pressing out the essential of a story is no easy task, everything seems sooo important… ) So, after this I started looking for some sort of guideline just to keep in mind that writing is also a craft and not some kind of artistic prose. I already had a good beginning, I had a nice starting point, I had the ending (yeahh!), I had great characters and after some struggling a nice big glue-story as over-everything-storyline keeping it all together. But the in between wasn’t so easy, so I examined the types of structures that exist and decided that the 8 Sequence might be helpful. I then reordered and reorganized my mindmap using this structure, added here and there and it seemed to fit well enough.
Like I stated before, I have and always had problems with filling or fitting my scenes for the second sequence. I don’t know if it’s the fault of the story or maybe this type of sequence simply doesn’t fit, but at least it gives me something to think about. I tend not to think of a structure as a corset, more of a guidance which helps me to remember about what the essence of each scene is.
Yes I know, I should read and analyze other books much more, but…, if the book is good I can’t stop reading it and I don’t want to spoil it with analyzing too much and if it’s bad I have no motivation to fuss around with it, more that I already did. I know, learn from the best and even more the worst, so to not repeat their errors No excuses…