As eyepro is compulsory, Boots are just common sense and I just don't like face masks outside of CQB sites, I am going to say Hat, scarf and load carrying gear. I was going to say Gloves as the 3rd item but tbh. my hands just get too hot so I tend to ditch them and take the risk of a bit of pain.
Getting shot anywhere in the head or neck hurts and a scarf can be used as face protection if you do get a bit close to the enemy.
For a hat, I recommend a boonie, they provide good enough protection and break up your silhouette a bit. Helmets provide more protection but tend to stand out like a sore thumb unless you add scrim. A hat with a brim also helps shield your ears a bit and is good sun or rain protection. My main 3 hats are an Aussie slouch hat (looks amazing but not the most stealthy), British airborne steel helmet with lots of scrim (bloody heavy and makes a very nice noise if someone gets a headshot) and a resin brodie helmet (only cost me £8 and is great in the rain).
My preferred scarf is a scrim scarf as it allows better air flow and can be draped over your head as a mini camo net if you want to hide. A shemagh is the other option, it will provide better protection and can be tied to provide face and ear (hits to the ears hurt as well) protection as well as protecting the neck.
There are lots of options for load carrying. Molle plate carriers or vests tend to be most popular but I find these get quite hot and there is a tendency to not feel hits when wearing them. Chest rigs belt kits and drop legs are other ways to use molle pouches that offer slightly less space but better ventilation. As a WW2 airsofter though, I have to put a word in for webbing. Webbing is cool (literally). When adjusted well, webbing will allow you to carry a lot of stuff really comfortably without acting as an additional layer of clothing that keeps you hot and prevents you feeling hits.