Dtac is the only vendor which has cut its fees more than 15% on average.
NBTC Policy Watch, a project under the Thailand Research Fund, a public organisation, held a seminar on its findings on Monday.
Pornthep Benya-apikul, a professor at Thammasat University's Economics Faculty, said since mobile operators began to offer 3G in June 2013, the problems faced by consumers remained the same since they had yet to be addressed.
The major concern is fees, which have not declined by at least 15% as required by the National Broadcasting and Telecommunications Commission (NBTC).
While Total Access Communication Plc (Dtac) has cut its fees by 24% on average from June 2013, the other two major operators have not fully complied.
True Corporation (True) has reduced its prices by 14% while Advanced Info Service Plc (AIS) has trimmed its fees by 8%.
Monthly packages offered by these operators have not changed much. In fact, AIS has not made any change at all to its monthly packages since June 2013.
Mr Pornthep said another issue was the reference price set by the NBTC, which was inconsistent with the industry's current pricing methods.
The operators bundle voice and data in packages, making it impossible for users to buy each service separately.
This makes it impossible for them or even the NBTC to check which service is overpriced.
"The NBTC often says the 3G prices have fallen more than 15% but it has never disclosed how it singles out each service from the packages, what method it uses in calculating them or what data they are based on," Mr Pornthep said.
He noted the packages of which prices were cut the most are often unpopular ones such as those for heavy internet users of 1GB or 2GB a month.
Nineteen of 24 groups of Dtac users enjoy cuts of more than 15%, most likely because Dtac does not bundle its services, allowing consumers to mix and match voice and data services that best suit their usage.
The flexibility means Dtac's post-paid 3G service comes in 24 packages, compared to five offered by True and AIS.
Mr Pornthep also said voice calls in excess of the quotas allowed in the packages cost more than the ceiling set by the NBTC at 0.99 baht or 0.82 baht for 3G.
This is a long-standing problem that has never been addressed, he said.