Farmworkers were caught on undercover footage jabbing pigs with pitchforks and repeatedly kicking them.
Some of the animals can be heard squealing in pain as they were booted in the face by Troy Wagstaff, 30, Artis Grogprkevs, 32, and Gavin Hardy, 39.
The pigs were hit with pitchforks by the men, two of whom could be heard laughing as a terrified sow attempted to leave its pen.
Some of the pigs also had gates shut in their faces, with disturbing pictures showing scars and bleeding wounds on the bodies of the vulnerable animals.
The trio were secretly filmed in April last year by the animal rights group Animal Equality as they worked at the Fir Tree Farm in Goxhill, North Lincolnshire.
The group had received an anonymous tip-off about the abuse.
Dr Toni Shephard, the UK director of the group, told how the men showed an “absolute lack of compassion” for the animals.
He said: “This is some of the worst abuse that we have seen – it was relentless.”
In some of the footage, the pigs are shouted at by the men and abused seemingly without provocation.
Supporters of Animal Equality and other animal rights groups were outside Grimsby Magistrates' Court as the men appeared before a district judge.
Hardy of South Killingholme, North Lincolnshire, pleaded guilty to causing unnecessary suffering to protected animals, an offence which Wagstaff, of Grimsby, and Grogprkevs, of Goxhill, had both admitted at a previous hearing.
The three men will be sentenced at the same court on 28 February, and were warned they could face custodial sentences.
Dr Shephard added: “We now expect them to receive the maximum prison sentence possible for this disgusting abuse.”
The trio, who no longer work at the farm, were all granted unconditional bail at the hearing.
Speaking outside court, Leon Plunkett, co-administrator for the Hull Animal Rights Association, said the only way to stop the sort of abuse the men carried out is if “everyone moves towards a vegan lifestyle”.
He added: “The only reason these industries exist is because people pay for them. Every time people go out and they purchase the body parts of these animals, they’re funding the industry. While slaughterhouses exist, it leaves it open for these animals to be abused.”
Additional reporting by Press Association
