Disturbing footage shows Troy Wagstaff, 30, Artis Grogprkevs, 32, and Gavin Hardy, 39, hitting the animals with pitchforks at Fir Tree Farm in Goxhill, Lincolnshire.
The men were taken to court after the Animal Equality group installed secret cameras at the farm in April last year.
Appearing at Grimsby Magistrates' Court on Thursday, Hardy, of Greengate Lane, South Killingholme, north Lincolnshire, pleaded guilty to causing unnecessary suffering to protected animals.
Both Wagstaff, of Chantry Lane, Grimsby, and Grogprkevs, of East Marsh Road, Goxhill, had both previously admitted the same charge.
The three men were granted unconditional bail, to be sentenced at the same court on February 28.
Speaking after the brief hearing, Dr Toni Shephard, UK director of Animal Equality, said the defendants had shown an "absolute lack of compassion" for the animals.
She added: "We're thrilled that all three of these violent workers have pleaded guilty.
"They've admitted that they were abusing the animals in their care and we hope now that they will feel the fullest force of the law and that all three of them will be sent to prison for these incredibly violent acts on vulnerable animals.
"This is some of the worst abuse that we have seen - it was relentless."
Leon Plunkett, co-administrator for the Hull Animal Rights Association, said the only way to stop such abuse is if "everyone moves towards a vegan lifestyle".
He added: "The only reason these industries exist is because people pay for them."