The dispute centres on the use of NHS Professionals to manage overtime.
Unison said staff could earn less for some overtime shifts than they would on a normal shift, with some workers missing out on as much as £80.
The union’s North West regional organiser Kevin Dolan said the trust had given “no indication that it’s willing to re-enter negotiations”.
Dolan also said operating theatre services were “chronically under-staffed” and staff were choosing to work extra shifts to cover shortages.
Unison said the new walkouts would take the total number of strike days since June to 10 if no agreement was reached.
Ahead of the August strikes, the trust declared a critical incident to help it secure extra support and keep urgent surgery running. It said some procedures would be postponed.