Is it permissible to marry the daughter of the elder maternal uncle while having been nursed by the younger maternal aunt, and does the presence of three children between the maternal uncle and the maternal aunt who nursed him have an effect?

The original ruling is that the daughter of the maternal uncle is not among the Mahrams (unmarriageable kin). Your maternal aunt suckling you does not make it unlawful for you to marry her daughter. However, if you were suckled by your grandmother (your maternal uncle's mother) along with your maternal aunt, then you become a brother to your maternal uncle through suckling. Consequently, it becomes unlawful for you to marry his daughters, because they are the daughters of your brother through suckling. The condition for suckling to be prohibitory is that there must be five full, satisfying sucklings.