Well, giants in future warfare ! The first thing that came to mind was the Engineers from the movie, Ridley Scott's Prometheus. That or the Warhammer 40k Space Marines who perfectly fit the description of armored giants that are few in number, but great in strength (compared to the mundane imperial guard from the same universe). Big benchmarking opportunity here. Oh and also Goliaths from XCOM game series.
And regarding giants in medieval era, oh boy. Lord of the Rings, any medieval fantasy with giants, and so many more. Might be worth looking into, if only for random ideas.
Now, on to the real answer:
After a few minutes of searching, I found this question and the associated answers. Although they talk about more medieval to renaissance-level of tech, much of it is still relevant: the weapons might have changed, but the tactics...not so much.
So, an updated version of the excellent answers from Deepon GhoseRoy/Glorfindel, user3644640 and Seallussus:
I divided these in two parts due to the fact that they often use more of a particular advantage the giants have: mobility and strength. Note that these are not exclusive, it is juste the most important attribute for said role
Your Giants are, well, giants. That makes them more powerful, intimidating and apparently quite resistant to bullets than traditional soldiers. Give them close combat weaponry, and actually melee weapons teams might even become a thing. Who cares about puny rifle bullets when you can easily stave off 23mm autocannon rounds? Just close the distance an slaughter ennemy infantry with giant sawblades, energy warhammers or whatever you fancy that looks deadly and sounds cool. Or take your trusty oversized shotgun and poke holes into whatever isn't a tank.
Heavy weapons teams
Ah, big guns. Do I really need to explain? Stronger soldiers can carry bigger guns. You have it. They come with an upgraded mobility compared to whatever exists today, because said guns would feel lighter to them. It's all a matter of size.
Kind of a subgenre of heavy weapon teams, but specialized. Tanks are big, bulky, and quite resistant to damage. Unless you have the right gear at hand, preferrably wielded by 3 meters-tall soldiers, who will have no issue bringing any heavy gear/weaponry to the battlefield. Sentient missile-spraying mobile platforms for the win !
A copy and paste from user3644640's answer. It is on spot:
That amount of size does not matter that much if creeping on forests camouflaged. They could carry more equipment. Behind enemy lines there is no supply so everything must be brought by people carrying it. More resources translates to longer missions or heavier equipment. Heavier equipment can be stronger radios, more ammo, maybe even a machinegun. Ambushing with a machinegun would be insanely strong deep behind lines where enemy is not adapted to the possibility. Your basic special forces story but when back to barracks a bit similar situation to above mentioned things.
Imagine you have to fight in a dense jungle. No roads, too many trees, and you have to stay discreet, so no clearing space for helicopters and drops are too conspicuous. You need to take whatever you need with you. Conveniently enough, you have huge sherpas at your disposal, who will carry a ton of stuff for you ! Better than pack mules, since they can fight too.
Due to their size, your giants will surely be faster than most soldiers since they have bigger legs. They can move around the battlefield quicker, climb stuff higher, see farther than the others. They would be great for harrassing troops provided they are in a terrain that vehicule won't be able to travel through (in which case they would lose the speed advantage necessary to skirmishers).
Go wild ! why not make these giants off-road team transports, they could easily carry three or four soldiers with a dedicated harness or something. Or you could build giant-carts and have modern versions of war chariots. That would be both awesome and terrifying. Ram 'em while your riders are all guns blazin' !
Support, Relief and Kidnapping
a bit of an even mix between speed & strength
Giants are strong, and they are quick on their feet. They will carry injured soldiers to safety whilst being able to protect themselves. If you need to extract a friendly VIP, or kidnap an ennemy VIP, they are the go-to. Quick, resilient, strong. Carrying a random guy in one hand while holding a gun in the other is no problemo for them.
Now, on to their weaknesses
Not particularily asked in the question, but it may very well help make it more elaborate.
First weakness: Well, giants are big. They will be obviousely weaker in tight spaces. Got a giant on your tail ? Jump into the sewers. It will be a Tom & Jerry game between giants and infantry, especially so in urban areas.
Second weakness: they are big. That means, they are bigger targets. You might alleviate that by giving them superhuman reflexes and agility though. It is your choice.
Third weakness: again, they are big. As in, they will be a lot more susceptible to fall damage. Pitfall traps will be deadly to them. Consider giving them exoskeletons if you don't want them dying from soap-related accidents.
Fourth weakness: from what I could read, their limbs aren't protected by the ridiculous shock-absorbing armor. The ennemy would most certainly capitalize on that and train attack dogs (preferrably oversized as well) to attack your giants, or at least pin them while the regular army does its job. Or prepare traps especially for them. Douse them chemicals that would either burn them, poison them, or be especially flammable. No limbs, no threat. To prevent that, give them at least armguards and legguards, and a flexible suit. That would help with the dogs and chemicals.
There are surely more that I couldn't see, I'll edit that if someone has any ideas.
A small addendum, very much facultative as this question isn't tagged "science-based" or "reality-check". Your giants, because of the square-cube law, will have either enormous energy needs (as in, they'll eat a LOT) or might be pretty slow, in a lot of ways. They would also be quite have quite a bit less stamina than regular humans (bad for skirmishers for example). But you can handwave it all as you see fit.