Now for my controversial opinion that may change at any time because that's how humans work.
It is true that each human has a "sex", which is determined at birth and probably can't be changed (Although the UK will allow you to change the sex on your birth certificate if you've been publicly trans for a number of years.)
But in my opinion that shouldn't affect how you address a person. Which is what gender usually ties into.
With regards to pronouns, I go by a "Call them whatever they want you to call them", because it's respectful and you don't harm their emotions with every word you say. This sounds fair enough right? If someone asks you to call them a girl, boy, whatever, you should probably just do it. Mistakes are human, outright refusal to accept someone's wishes is not. If you don't want to run afoul of the "you just assumed my gender??" stuff, neutral terms work well if you're unsure, however I doubt anyone would witch-hunt you for making an assumption and respectfully accepting the correction. It's always when the person refuses to accept the correction and is stubborn where problems arise.
Now let's throw this into the topic of a third gender. Honestly, does it even matter? It's not like we're trying to keep tabs on all these "genders" and make a list of them (Although some people have). If someone asks you to refer to them in a certain way (Be it as agender, genderfluid, or any word they've decided to use), the sensible thing to do would be to just go with that. Respect their wishes, and nobody has a problem.
Now of course there'll be someone who'll bring up the Apache Helicopter meme, and will complain about that. Honestly, anyone who says something like that is probably joking, but if they're honestly serious then what's the harm in respecting it? Everyone is different, and deserves to be addressed however they like.
If you're still reading, an analogy I've thought of on the spot is names. If someone asks to be called by a totally new name (forgetting genders for a second), you wouldn't exactly go "no i want to use your old name forever, you're stuck with it." would you? Then why can't this apply to pronouns and gender and stuff, if someone says "please refer to me with neutral words from now on", just do it. There's no harm, and it keeps everyone happy and civil.
I mean sure if you want to get petty then yeah everyone has a "sex" which they're kinda stuck with, unless you count changing genitals as changing sex (Which I think the UK government does, to the best of my knowledge), in which case good on you. But what you call someone and what genitals they have are two different things. The latter is usually referred to as gender and is tied to specific behavioural characteristics, while I believe its just a way of referring to someone, and people can ask to be called whatever they like, it's their life and their choice.
I will not write a tldr for this huge post because i think you should just read it all, and yes I did focus on ways of addressing people rather than what gender really means, but that's how I see it.
If you're going to reply, I'd ask for you to do it respectfully and calmly (Which most of you seem to be doing), so we can discuss it like adults. Nobody likes someone who flings their opinion at someone as if it was fact. The above is my opinion and mine alone, I will not deny you yours.