The women were reportedly tied up, gagged and assaulted by unknown attackers in their parish home in Matias Romero municipality earlier this week.
This attack comes nine months after a similarly brutal attack on the parish priest in the same village.
The Bishop of Tehuantepec condemned the attack on the nuns from Apostolic Missionaries of the Sacred Heart and the September 2014 attack on Father Victorino López Nolasco: "We observe with sadness and indignation that this violence has not been combatted in a timely or effective manner. Indifference, under-handedness, and on occasion the complicity of public institutions and society itself have tolerated and fostered the growth of criminality."
"We cannot continue to avoid our responsibility to eradicate this evil that will end by destroying social development and even the lives of many people, including the lives of our young people... For this reason as citizens and as believers we take a stand against all kinds of violence. We ask the Authorities, who are responsible for protecting the security of citizens, that they do their job to eradicate this social evil which appears to be expanding like a plague in our region."
CSW's Chief Executive Mervyn Thomas said, "We were deeply saddened to learn of the horrific attack on the three elderly nuns in Oaxaca earlier this week.
"We join the Diocese of Tehuantepec in their call for a thorough investigation and for those responsible to be held to account. The situation is very complex and all of Mexican society, including communities of faith, is affected by the culture of impunity.
"We remain deeply concerned by the impact of the violence, and in particular impunity, on freedom of religion or belief, and we stand in solidarity with religious leaders like Father López Nolasco, who continue to carry out their ministry despite the very real risks."
According to Vatican figures, more priests were killed in Mexico last year than anywhere in the world.