The United Nations, International Committee of the Red Cross (ICRC) and Syria's Red Crescent (SARC) said in a joint statement that simultaneous deliveries reached the towns on Monday.
They said fuel had entered Fuaa and Kafraya, which are under rebel siege, and Madaya, which is under a government siege.
Food and medicine was also delivered to the rebel-held town of Zabadani, which was not included in similar aid deliveries to Fuaa, Kafraya and Madaya this month.
The statement said however that a joint delegation of aid officials was not able to enter Fuaa and Kafraya to carry out assessments of humanitarian needs.
"The joint team had to postpone the mission to Fuaa and Kafraya upon receipt of reports from armed groups that more time was needed to finalise security arrangements in areas under their control," the statement said.
All four towns were part of an agreement last year to end fighting and allow the entry of humanitarian aid.
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But access to the towns has been fraught, and there have been reports particularly from Madaya that civilians have starved to death under siege.
There have also been reports of deaths from lack of medical supplies in Fuaa and Kafraya, though the government has been able to airdrop some supplies to the towns, a capacity rebels do not have.
Some 42,000 civilians are believed to be in Madaya, with fewer than 1,000 in Zabadani and some 20,000 in Fuaa and Kafraya.
After the September deal, an initial aid delivery was made, but no subsequent assistance was allowed in until January 11, after reports of deaths in Madaya raised international concern.
A second convoy of food and medicine entered Madaya, Fuaa and Kafraya on January 14, but the UN, ICRC and SARC have called for continuous access to all four besieged towns.
Aid groups have also warned that residents suffering from malnutrition and illness need to be evacuated from the towns.
Last week, UN Secretary-General Ban Ki-moon urged all sides in Syria's war to protect civilians and warned that "the use of starvation as a weapon of war is a war crime."
More than 260,000 people have been killed in Syria since the conflict began in March 2011 with anti-government protests.