Christopher Weatherhead, 22, who operated under the alias, Nerdo was described as the leader of the group.
He was given an 18-month sentence at Southwark Crown Court, London, after being found guilty of conspiring to impair the operation of computers between August 1, 2010 and January 22, 2011.
Ashley Rhodes, 28, of Bolton Crescent, Camberwell, south London, admitted the same charge and was jailed for seven months.
Co-defendant Peter Gibson, 24, of Castletown Road, Hartlepool, was deemed to have played a lesser role in the conspiracy, which he also admitted, and given a six-month suspended sentence.
Jake Birchall, 18, from Chester, will be sentenced later. He had also admitted the conspiracy.
Weatherhead, who used the hacker name Nerdo, was convicted last month of participating in a "distributed denial of service" (DDoS) on the financial services websites.
DDoS attacks cripple websites by flooding them with requests until they crash.
One attack on PayPal lasting ten days cost the company £3.5m