"Considerable challenges lie ahead, and we are concerned that the arrangements agreed to date are not realistic or sustainable," said the three countries in a joint statement Friday.
"Given their past leadership failures, South Sudanese leaders will need to behave differently and demonstrate commitment to peace and good governance," added the statement.
The three countries stressed that they now expect to see a change in the situation on the ground, beginning with a further significant reduction in violence, and all parties taking measures to allow full humanitarian access.
However, the three countries expressed support for the engagement of the region in the recent Khartoum-based negotiations on outstanding governance and security issues and acknowledged the role of Sudan in hosting these negotiations.
South Sudan's conflicting parties, on Aug. 5, signed a final deal in the Sudanese capital Khartoum on power-sharing and security arrangements.
The deal was signed by South Sudanese President Salva Kiir, major opposition leader Riek Machar and representatives of other South Sudanese opposition factions.
It stipulates that Kiir will continue his post during the transitional period, while Machar will be the first vice president among the four vice presidents from different political parties.
Under the agreement, the transitional cabinet would be composed of 35 ministers, including 20 ministers from the government, and nine from Machar-led Sudan People's Liberation Movement-in-Opposition (SPLM-IO).
The deal stipulates a transitional national legislative body would be composed of 550 members, with 332 from the government, and 128 from the SPLM-IO.
South Sudan has been witnessing a civil war since December 2013, which has left about 10,000 dead and millions of others displaced.