With three tanks the enemy has a huge amount of health and shields: they push together, absorb a lot of damage and take space from you. But three tanks also means less damage, so the fight comes down to who lasts longer and who deals more sustained damage.
How to spot it on the loading screen
Three Vanguards on the team, for example Hulk, The Thing and Doctor Strange.
A single duelist, often a high-damage one like Hela.
Healers who sustain behind the shield, like Luna Snow or Invisible Woman.
Why it's dangerous and where its weak point is
Against that much health, sustained-damage duelists and shield breakers perform much better than those looking for quick kills. Since the enemy deals little damage of its own, you can afford more damage in your comp.
The 2-2-2 comp the tool recommends against that enemy: Punisher breaks shields and brings sustained damage, Hawkeye punishes from afar, and Adam Warlock + Gambit provide high healing for the long fight.
The exact recommendation changes depending on the enemy's heroes and the ones your team already picked. For your real match, load both teams in the tool and it builds the full comp for you.
Tanks: You don't need to match their three tanks: two is enough. A shield (Magneto) to stop their push and a tank who fights well up close (Captain America).
Duelists: Sustained damage and shield breakers: Punisher, Hela, Hawkeye. Against that much health, constant damage is worth more than burst.
Healers: High-healing healers to win the long fight: Adam Warlock, Gambit, Luna Snow.
Common mistakes
Bringing duelists who need quick kills: against three tanks they almost never get them.
Shooting the tank with the shield up instead of going around it or breaking it.
Fighting in tight spaces, where three tanks together are harder to split up.