First: memorizing these terms is not particularly useful for understanding biology. I hope you're studying somewhere where these sorts of questions aren't going to show up on a multiple-choice exam.
I'll be very brief to give you a starting point; I'd recommend digging into each of the Wikipedia articles I link to for more if you need.
Phylogenetics involves inferring the history of common descent among organisms. The result is something that looks like a "tree", just like a family tree, where at one point in the tree (the "top", or often the right-hand side) you have a list of present-day organisms, and branches in the tree represent a past common ancestor. Phylogenetics may also quantify distance between related organisms.
Cladistics assigns names to the branches of the phylogenetic tree.
Both of these are examples of Systematics, which I would describe as a more general name for the concept of organizing organisms according to common descent.
Whenever you do phylogenetics or cladistics, you're doing systematics. You probably can't do cladistics without doing some level of phylogenetics along the way, but also sometimes the emphasis may not necessarily be on the finer details depending on the goal: you wouldn't need to know who all the parents are when organizing a list of someone's grandchildren. Similarly, you can distinguish between different clades without knowing all the relationships within each clade.
In many cases, all three terms will apply simultaneously, and the word you choose might not be that important or might indicate some subtle distinction in goals.
