The increase is due to the growing numbers of cabin crew who want to work as normal, the airline said.
BA will increase its Heathrow long-haul schedule to more than 80% of flights, up from more than 70% this week and 60% in the first strike period.
It will also increase its Heathrow short-haul schedule to 60% of flights, up from more than 55% this week and more than 50% in the first strike period.
BA flights at Gatwick and London City airports will remain unaffected by the industrial action.
The airline will continue to lease up to eight aircraft from other UK or European airlines to supplement its short-haul schedule at Heathrow.