An independent claim will always encompass all of its dependent claims. That means that something infringing a dependent claim will always infringe the independent claim too. A dependent claim is just a narrow version of the independent claim.
So let's imagine you would invalidate a dependent claim, e.g. a+b+c. Then your abc would still infringe the independent a+b claim. Because a+b means a+b+ anything (x). And that's the catch. Abc is prior art for ab+x and can invalidate the ab claim.
It it's impossible to invalidate a claim that only specifies a broader claim without invalidating the broader claim. Dependent claims exist for a similar case. Because abd would invalidate ab, abc is used as a fallback position.
There is one exception that's actually not an exception but could lead to confusion. If in the past abc was thought to be impossible without a and b and c you can invent ac. By the above logic that would mean that abc infringes your ac claim. But that's just a limitation of this simple example structure. Ac in this case is actually abc-b or only ac“.
If you tried to make abc a dependent claim because it's more narrow than ac, that wouldnt work because abc-b+b doesn't make sense and neither does “only ac“ + b.
To summarise, any protection that a dependent claim grants may not be broader than the protection the independent claim grants. And an example of prior art that falls into the protection of a claim is enough for invalidation, so by definition invalidating a dependent claim must invalidate the independent claim.
"a flexible structure with three specific, clearly differentiated areas of a different nature"
but I reduce it to 2 parts whilst keeping the like for like functionality, is this infringement?
If the claim clearly states that the parts have to be present then no, unless you're running into equivalency. That's basically infringing by substituting a part of the claim without functional differences. But that's a different, very complex topic. I just mention it because shipping around the wording of a claim without shipping around the meaning will get you into problems with that.