Most of the fights are 3vs7 and, looks like, at any time you are ready to take down the boss, they all summon 3 more and then 3 more/

Hell, is common that they summon reinforcements, on a 3vs4, on their first turn!
I spent most of the time, running around silly dwindling their numbers, hit the boss a couple of time and out of nowhere you get gank by reinforcements.

Seems a bit dumb to be able to stay in an indefinite amount of time in a boss room killing enemies.
Its a bit counterproductive being around that much number of enemies by a drop of a hat.

Is there a difficulty that just stops some bosses from summoning infinite reinforcements? Are the reinforcement that infinite?

The goal is to overcome the boss through whatever means necessary. The reinforcements don't need to be defeated.

In fact you can simply nuke the boss and win! This is obviously only applicable for boss fights, other fights require you to win against all enemies.

Echoing what AverageBear said: the boss fights require you only to beat the boss. Generally I'll try to quickly defeat the minions that are flanking me and then move so that the boss meets my 3 characters on an open battlefield. If you do it right you'll have a couple turns with just your 3 against the boss, before the reinforcements make it across the map to attack you.

If one of your characters has a skill that draws aggro, use it and then focus all attention on taking down the boss. Wasting time on his reinforcements will cause you to get beat.