Three essentials? Well we can exclude eye protection, because that's compulsory, although in picking that, I'd personally recommend small mesh goggles, which don't fog up. But for the three essentials, I'd say...
Lower face protection: Which is only compulsory if you're under 18. I don't actually like having to wear it, but doing so is sensible, since you get plenty of shots aimed at your head, and a BB will smash your teeth out if it hits them; seen it happen loads of times to people.
Neck protection: An Arabian keffiyah (shemagh) will do a good job of protecting your neck and looks the part too, although if you are playing in woodland, a traditional white/black one stands out quite a lot, so if you can, get an olive drab one. A BB to the neck will hurt like hell and probably draw blood, and if it hits your adams apple, it will literally make your eyes water with how much it hurts. Failing that, any other sort of scarf will do as well. Scarves can at a push also serve as lower face protection.
Gloves: Your left hand (if you are right handed) will also take a lot of hits, since it is holding the foregrip of your weapon, and like with your neck, the skin is thin, so you will get real nasty blood blisters coming up if a BB hits a finger. Thin leather gloves of the type which modern military pilots wear are a good choice, as are the nomex ones which a lot of fighter pilots wear, since they are a gauntlet style, so they also protect your wrists as well. Pilots have to operate radios and other small switches, so that type of glove does not interfere with your ability to operate fire selectors on weapons, or speed loaders and such, and as will all military surplus, you tend to get very good quality gear at a price which is acceptable.
Honourable mentions. Other contenders for essentials could probably include:
tactical vest/load carrying vest/plate carrier: The important thing with those is practicality rather than how cool you think it looks, in other words, is it easy to get stuff in and out of the pouches with your eyes shut, if it isn't easy to do that, it's no good, because when you are fighting, the fumble factor will ensure that you lose.
knee pads: You actually only need one, because you'll tend to kneel down to take shots on the same knee all the time, but in woodlands, your leg will end up damp if you kneel down just wearing combat pants alone.
helmet: A military helmet has good 'pose value', but it's a useful bit of kit in airsofting because it will protect you from a lot of painful hits, some of which will mark your face for days after, particularly if you get hit on the forehead, which again is thin skin and will bleed and swell up. If you have the kind of job where you have to deal with other people face to face, coming into work on monday looking like you have been in a fight is not a good thing. You can find airsoft replicas of military helmets for anything from a tenner upwards, so this doesn't have to break the bank. Avoid a genuine military steel pot helmet, yes they can sometimes be had cheap and are cool (I have a few of them), but they are intended to stop shrapnel from explosions, so they are built heavily and are not comfortable to wear all day long. At a push, a baseball cap will do for airsoft, but a helmet will do a better job.
Waterproof combat boots: Offering good ankle support and keeping your feet dry on woodland skirmishes are the two obvious benefits of these, and they can be had cheap from army surplus stores. Genuine military ones are always very decent quality and are designed to be worn for long periods, which is of course ideal for a day skirmishing out in wet weather
A speed loader: The little spring, pump-operated speed loaders (which often come in the box with a gun) are small enough to fit into a tiny pouch on your tactical vest, and mean you can stay out in the field a bit longer instead of tramping back to the safe zone, since most of them will hold a couple of hundred BBs.
Water bottle: You will get hot when skirmishing, and can dehydrate, so having a drink on your person is a good idea.
Tac light: Fairly essential in CQB, one which can be operated by a thumb switch on your rifle's foregrip is the best sort, because you only want the light on when you need it, so you don't give your position away.
Rifle sling: The slings which come with guns are almost always cheap pieces of sh*t, which will fail and make you drop your weapon and probably break it, so a few quid spent on a good sling is a sound investment. And you really do not want to be carrying a weapon around all day long, especially a full metal one; even the lighter plastic weapons start feeling pretty goddam heavy after a few hours.