Describe at least 3 monsters we see in film or television today. Consider traditional monsters (like Grendel) but also monsters who threaten the well-being of society.

Monsters take many forms in media today. A popular classic is the zombie, which typically represents a number of societal phenomena: consumerism, loss of individualism within society, and a deep fear that humanity is made up of mindless monsters. Zombies are terrifying because they threaten the structure of society (in zombie media, society is often destroyed during the monsters' rise); in this way, they represent a fear of humanity's ability to destroy the society it has created.
Another classic is the werewolf, which, of course, symbolizes the "beast within" and the inability to control one's animal, aggressive urges; a similar, more specific example of this is Dr. Jekyll and his transformation into Mr. Hyde. Grendel can be considered a monster within this category. He is given very little physical description, but is used as a foil to Beowulf, therefore representing the antithesis of the great, noble warrior.
This source from Washington State University gives more detail on Grendel as a monster, as a foil to Beowulf, and monsters in general: WSU MONSTERS FRONTPAGE

As this question depends on your own opinion and addresses social well-being, you could approach it from several perspectives. One promising way would be to give a range of monsters, from literal to abstract. A traditional monster like Grendel could be any evil character from movies or television. Slightly more abstract could be the personification or amplification of threatening characteristics in more modern robot-type or cyborg characters. In the abstract realm, you might use an example of something meant to benefit society that has gone awry, such as the misuses of technology through hacking. Or, paying homage to Shelley’s complex presentation of Frankenstein’s monster and considering modern inversions of monstrosity, including making Grendel a hero, you could reverse that approach and highlight "the good in the bad."
For the first, zombies seem an obvious choice, as they threaten society through literally eating people, For the second, you could consider a figure like the Pretender in the Transformers series, or a classic example such as the Terminator.
For the third, large-scale hacking in data breaches can take on a monstrous scale, affecting all the users of a particular site, like Yahoo or eBay. For a reversal, in which technology is positively humanized, the 2013 Spike Jonze film Her comes to mind.