The three nations' military rulers announced last month they were abandoning the political and economic bloc in a blow to integration, after ECOWAS had pressured them to restore democracy.
The bloc's mediation and security council met in Nigeria's capital Abuja to discuss both their departure and Senegal, where a 10-month election delay has sparked opposition challenges.
ECOWAS Commission president Omar Touray said during opening remarks that "the hasty decision of withdrawal of membership of ECOWAS did not take into account the conditions for withdrawal."
He did not specify which conditions had been ignored.
Member states wishing to withdraw must give a written one-year notice, which the countries did not do alongside their initial announcement but have done now.
Touray said the bloc had prepared two memoranda on the issue for consideration including an analysis on implications for member states and the wider community.
A document was also prepared on Senegal.