Four out of five members of the telecom committee voted to ratify the bid outcome. Prawit Leesathapornwongsa, who has been opposed to the bidding structure, did not.
The three bid winners could secure licences within a week after paying half of the bid prices to the NBTC, which are due within 90 days.
The 15-year licences will be issued within seven days from payment.
The three bidders, Advanced Info Service's (AIS) Advanced Wireless Network; DTAC Network of Total Access Communication (Dtac); and True Corp's Real Future, each won their maximum quota of 15MHz of spectrum.
AIS offered 14.625 billion baht while Dtac and TrueMove offered 13.5 billion baht each. The sale of the 2.1-gigahertz spectrum came in at only 2.78% above the reserve price.
Telecom committee chairman, Settapong Malisuwan, spent more than three hours defending the auction's structure, terms and conditions before the vote was cast. The meeting was broadcast live to journalists.
At one point, Dr Prawit called the broadcasting a bad idea because sensitive issues had not been settled.
Col Settapong defended the broadcast, saying it was to ensure reporters got the facts straight.
Gen Sukit Karmasundara and Prasert Silpipat staged a brief walkout after Dr Prawit repeatedly raised concerns about the bidding's revised structure.
He was opposed to the revised design, which lowered the spectrum cap from 20MHz to 15MHz without increasing the reserve price.
The bidding design failed to maximise competition among the bidders who ended up paying only for the garnered slots plus low annual licence fees, Dr Prawit said.
Under the current concession contracts, the three operators were bound by the revenue-sharing system in which they would set aside about 25-30% of their income to the state.
Mr Prasert said it is time to move on, especially now that the bidding has been completed.
The broadcast was suspended for five minutes when Dr Prawit asked for the log files containing data from the auction.
Col Settapong then decided to call for a vote in which the four members ratified the bid result.
The NBTC will issue certificates to the three winners today. The regulator will hand over the money to state coffers as soon as possible, after deducting 20 million baht for the actual cost of the auction.
As the endorsement went ahead as expected, a group of people led by Sirisak Siriporn-udomsilp, a lecturer at Khon Kaen University, lodged a petition with Senate Speaker Nikom Wairatpanich seeking the removal of all 11 members of the NBTC. The petition was signed by 57,904 people.
Mr Sirisak said the NBTC members might have violated Article 11 of the Price Collusion Act of 2009 and Section 47 of the constitution which requires the NBTC to act in consumers' interests.
The watchdog might have also committed a serious breach of ethical standards in their handling of the 3G bid.
Mr Nikom said the Senate would take some time to verify the names before forwarding it to the National Anti-Corruption Commission.
In a related development, the Administrative Court yesterday threw out two petitions against the NBTC over its handling of the 3G auction.
The first petition, lodged by the Consumer Rights Protection Association (CRPA), sought disqualification of DTAC Network from the bidding on the grounds that the firm is allegedly a foreign-dominated firm.
The court dismissed the lawsuit stating that the association was not a damaged party. Moreover, its concern about foreigners' control over the frequency was mere speculation.
The court threw out the other petition, lodged by Adm Chai Suwanpharp, calling it invalid.
The complainant said that as a "creditor" the NBTC might not be able to fully protect consumers. The court noted that the NBTC is not a creditor and the bidders are not its debtors.