Thomas Harris, 27, from Ringwood, Hampshire, pleaded guilty to conspiracy to commit criminal damage at earlier hearings.
Christopher Potter, 20, and Maria Neal, 21, both from Seggs Lane, Alcester, Warwickshire, were also each sentenced today to 12- month prison sentences, suspended for two years.
Potter and Neal were also ordered to carry out 120 hours of unpaid work.
The three conspired to attack four branches in Hampshire in 2008 by daubing graffiti such as "murderers" and "scum" on doors and gluing post boxes and cash machine slots.
They also painted "ALF" - standing for Animal Liberation Front - on the side of the buildings.
In addition, they damaged a car belonging to someone they mistakenly thought had links to a company supplying HLS during the conspiracy, causing damage costing £7,500 to repair.
A van belonging to FedEx, another Stop Huntingdon Animal Cruelty (Shac) target, was also attacked by the extremists.
Roger Climie, prosecuting, said that Harris was in charge of Shac at the time of these attacks.
He is already serving a four-year jail term for conspiracy to blackmail companies linked to HLS in an attempt to close it down.
His sentence today is consecutive to the term he is already serving.
Mr Climie added that it was accepted that Harris was abroad at the time of two of the bank attacks.
Sentencing the three defendants at Winchester Crown Court, Judge Keith Cutler said: "Each of you have pleaded guilty to a conspiracy which, quite frankly, reflects an appalling period of behaviour."
He described Potter and Neal as "young and idealistic" and added: "I hope when you look at yourselves two years ago you recoil in horror and think 'What on Earth was I doing?'.
"I hope it will haunt you for the rest of your life."
Mr Climie said Shac had a facade as a law-abiding protest group but a number of members were involved in its secret but illegal activities.
He said: "This was an organisation where the numbers privy to the events taking place behind the scenes were comparatively small."
Barclays said it no longer had links to HLS but did through its asset management business when the attacks took place.
The hearing is the latest in a series of prosecutions of people linked to Shac.
In October last year, six activists, including Harris and his girlfriend Nicola Tapping, were sentenced for their part in attempts to close HLS down.
Other members of Shac, including founder members Gregg Avery, Natasha Avery and Heather Nicholson, were given lengthy jail terms in January 2009 for blackmailing companies linked to HLS.
The court heard that during the six attacks, the three defendants caused a total of £12,000 worth of damage.
All three defendants were also given Anti-Social Behaviour Orders (Asbos) to last five years.
Detective Chief Inspector Andy Robbins, who led the investigation and now heads the National Domestic Extremism Team, said: "We welcome the sentences passed today and hope that it draws a line under the campaign of criminality and intimidation carried out by members of Stop Huntingdon Animal Cruelty.
"These were carefully planned criminal offences carried out during the cover of darkness against organisations carrying out perfectly lawful business.
"The vast majority of animal rights campaigners conduct their campaigns peacefully - and the police service will always strive to facilitate this.
"However, as we have seen today, where an individual uses crime to further their cause, we will investigate and are committed to bringing offenders to justice."
Alastair Nisbet, head of the Crown Prosecution Service Wessex complex casework unit, said: "These defendants were not legitimate animal welfare protesters but criminals with no respect for the property of others.
"They damaged the car of a person who they mistakenly believed was working for a pharmaceutical company, several bank branches in the Hampshire area and a courier's van.
"While these individual incidents may not seem especially serious, they were a continuation of the wave of criminal activities started by the discredited organisation Shac - Stop Huntingdon Animal Cruelty - whose sole aim was to shut down Huntingdon Life Sciences Plc (HLS) by any means necessary, regardless of the law.
"HLS carries out pharmaceutical product testing which is not only licensed and lawful, but is required by UK and European legislation.
"These defendants intended to intimidate those who were going about their lawful business because they personally objected to the victims supplying goods and services to HLS, or using their services.
"Our society supports the right to free speech and to campaign peacefully, but where that behaviour is criminal, regardless of the perceived cause, a prosecution will almost certainly follow."
