The Australian Federation of Air Pilots said its members working for Network Aviation and QantasLink in Western Australia would strike from Wednesday to Friday over stalled wage negotiations.
Senior Industrial Officer Chris Aikens said pilots had had enough and the planned two-day strike would stretch to three days.
"Qantas management has angered our Network members by walking away from negotiations and, last week, taking previously agreed items off the bargaining table," he said on Monday.
"Our members deeply regret having to take this protected industrial action but are left with no other option."
Pilots walked off the job on Thursday for 24 hours sparking the cancellation of 35 Network Aviation flights.
Qantas Group said it was disappointed there would be a third day of strike action and plans for Friday's action would be put in place in the coming days.
The airline said affected customers would be contacted and they could make fee-free changes to their flights or request a refund.
The pilots' federation has been attempting to negotiate with Qantas Group for 18 months to replace a pay deal which expired in 2020.
"Qantas can readily resolve this by recognising that Network pilots are only seeking to achieve terms and conditions of employment that are commonplace amongst the Qantas pilots and the airline industry overall," Mr Aikens said on Monday.
"We remain keen and willing to meet with the company's management to arrive at some improvements in terms and conditions for the lowest-paid jet pilots."
Network Aviation said the pilots had previously been offered and rejected pay increases of more than 25 per cent, plus yearly three per cent increases, new allowances and greater roster protections.
The union has rejected the assertions.
The airline last week made an intractable bargaining application to the Fair Work Commission on Monday to force a new enterprise agreement with its pilots to break the deadlock.
Network Aviation pilots walked off the job over pay negotiations for 24 hours in early October, causing more than 40 flights to and from regional towns and mine sites to be cancelled.
"It's unfortunate that the (federation) has again chosen to target the travel plans of West Australians and the mining sector as part of its industrial campaign," Qantas said in a statement.
"We have not walked away from negotiations. Given the impasse, we were left with no other choice but to ask the Fair Work Commission to arbitrate to bring this to a close."
The airline, which is wholly owned by Qantas, is WA's premier charter company for the mining industry and operates hundreds of flights a week.
It also employs local pilots for the carrier's regional arm, QantasLink.
More than 90 per cent of its 250-plus pilots are members of the pilots federation.