Paraje Tres Cerros is a low altitude hilly natural area surrounded by cattle fields and characterized by three isolated rocky formations of about 150–180 m a.s.l. located in the Corrientes province of Argentina. Their topographic and environmental conditions are unique across the whole Mesopotamian littoral of Argentina and, therefore, can be considered as a biogeographic island that hosts several endemic species of plants and animals. The lack of knowledge of the mygalomorph spider species of the area led us to conduct a field study at Paraje Tres Cerros with the objective of surveying these spiders. From this survey, we described two new endemic species, Stenoterommata isa sp. nov. (Pycnothelidae) and Catumiri sapucai sp. nov. (Theraphosidae); and we recorded Xenonemesia platensis for the first time in the Corrientes province. In addition, we described the sexual behavior of Stenoterommata isa sp. nov. and Catumiri sapucai sp. nov. for the first time. We presented distribution maps for the Argentinean species of Stenoterommata, for the genus Catumiri as well as for Xenonemesia platensis. Stenoterommata isa sp. nov. constitutes the eighth known species of the genus in Argentina and according to its distribution is the only exclusive species recorded in the Corrientes province. Regarding its sexual behavior, we obtained one mating, which consisted in touches the female legs with legs I and II by the male, who starts the courtship by beating the female’s cephalothorax and sternum with legs II and palps respectively. Then the male clasps with the first pair of legs between the palp base and chelicerae of the female and elevate her to reach the genital opening for the palpal insertions. Catumiri sapucai sp. nov. is the third species described from Argentina with a new spermathecae shape with two additional elongated digitiform domes external to the inner receptacles. We obtained five matings for this species, two of which involved males contacting females without an evident courtship. For the remaining three, the males initiated courtship by performing a few quick body vibrations. All males achieved the typical copulation position observed in most mygalomorphs and made about 3 to 5 palpal insertions, with the exception of one case in which spiders lost equilibrium and separated from each other. The presence of Xenonemesia platensis at this area is based on one adult female, thus further campaigns to collect more specimens will lead us to confirm this record or to better elucidate its taxonomic identity.