In the third crusades what were the band of assassins who killed william de montferrat and others?

Answer 1

Assassins [Arabic: hashshāshīn] were a sect inside Shi'i Islam in the period from the 11th to the 13th century. They were known for their suicidal murders especially during the times of the Crusades.
The reported mode of operation is that the Assassins performed their deeds under strong intoxication from hashish with the promise of immediate entry to paradise. That paradise that had been staged for them during their training at the Alamut - - a paradise of sweet food and wine and beautiful and willing women.
They were the first "Terrorists"
Terrorism became a religious duty. The Assassins built a number of strongholds all over Iran and Iraq. They had a system secret agents positioned in enemy camps and cities. The Assassins often worked closely with certain leaders of Muslim states, as their services were attractive: no one else was better able to kill important persons in enemy states.
One of the most important Muslim allies of the Assassins was the Seljuk ruler of Aleppo, Ridwan. Through this cooperation, they were able to establish themselves in the Syrian mountains, where several fortresses were erected.
Even Saladin - -who was the Arab leader during the time of the Third Crusade - - came to treat the Assassins as allies, although he intended initially to eliminate them. The reason for this alliance, was that Saladin, following two assassination attempts, feared for his own life. It was safer to be an ally of the Assassins than an enemy.

I'm not sure which William de Montferrat you are referring to.
For the time period of the 3rd Crusade it would be William V of Montferrat (1115 AD – 1191AD), also known as William the Old. He died in Tyre ~ 1191 at the time of the 3rd Crusade. I didn't know that the Assassins got him.

Answer 2

as far as i know, there is no assassin that have killed them. the only thing i know is that they were captured by their enemies at that time.