no one sees the dragons as the extremely OP doomsday pieces of war they are
Because they're not. They are baby dragons, and they're not being trained properly because Dany has no idea how. Two of them have even been chained away for a long time.
The last time we saw Drogon, who's the biggest, he couldn't carry a boar/horse and could barely carry Dany.
From what we hear of Aegon's conquest
Aegon's conquest is absolutely nothing like what Dany will do. Aegon, Visenya, and Rhaenys were three dragonriders, had trained their dragons from the moment they were born. At the time of the conquest Balerion was 120, Vhagar 50, and Meraxes over 25 years old.
Dany's dragons are three.
dany could land with about 3000 men and her dragons and take weseros
No.
Once she recreated the field of fire
There will not be another Field of Fire, since no Lord worth his salt will keep their army so tight together that a dragon could cause maximum damage. Anybody in Westeros who will encounter dragons now will simply go with what they know has already worked before: dornish guerilla tactics.
And that's not even touching on the fact that the Field of Fire took place during summer. Dany's dragons will have to fight in/against a Westerosi winter, possibly even a Long Night.
having 1, let alone 3 dragons would be enough
Well, Dany has one. What if Bran, Victarion, or Brown Ben successfully steal one of the other two?
and how big they were getting.
Drogon at the end of ADwD is as big as a horse. A winged horse, but still.