Are you
One (like the number/word)
Firstly, I am one word long.
Well, it's one, it's the length of the word one. That's why the sentence is oddly worded.
Secondly, you will need the first three things.
First three things... three letters.
You'll need a handful of guesses^1 to figure out what I am.
A handful of guesses, and then one. (this might be overthinking kicking in but maybe not.)
But even if I tell you, I might be lying, mate!
Well, this seems to be a play on "I" being like a "1"?
Hint 1:
A play off of how the number one is in every part of everyday life, so it's unlucky if you don't see it?
Hint 2:
Pay attention to ONE of the tags. Here's one again.
I could be completely wrong here though.
Another guess:
Are you
Familial
Firstly, I am one word long.
Familial is one word long
Secondly, you will need the first three things.
I unscrambled all the third words without the hints and it came out to this.
You'll need a handful of guesses^1 to figure out what I am.
It's a word that rarely is used.
But even if I tell you, I might be lying, mate!
Maybe just something to show you the "if"
Hint 1:
If you don't have a family (familiar means relating to or having the characteristics of a family; "children of the same familial background"), you're viewed as unlucky.
Hint 2:
This tells you to look at the steganography.
This is also probably incorrect.