Narongchai Akrasanee, Korbkan Wattanavrangkul and Dr Rajata Rajatanavin stepped down, said Mrs Norarat, who informed the NLA of their resignation letters.
Reasons for the resignations weren't announced, but Mr Narongchai, an adviser to the National Council for Peace and Order, reportedly is tipped to be energy minister in Prime Minister Prayuth Chan-ocha's cabinet.
While the NLA is six short of its designed number of members, only 97 are required for a quorum.
The resignations follow that of Maj Gen Intarat Yodbangtoey, Gen Thawatchai Samutsakhon and Aziz Pitukkumpol, the Chularatchamontree.
Maj Gen Intarat was a deputy leader of the now-dissolved Matchimathipatai Party. As a party executive, he'd been banned from politics for five years.
Gen Thawatchai was a member of the Chart Pattana Party during the past three years and, thus, ineligible for the NLA under the interim charter. Mr Aziz, a Muslim community leader, declined for religious reasons.