Speaking after naming the Springboks team to play Portugal on Saturday in a one-off Test in Bloemfontein, he said the trio will need "between four to six weeks to recover from injuries".
Erasmus said hooker Marx and lock Mostert have leg injuries, but did not specify what had sidelined flanker Du Toit.
South Africa begin their Rugby Championship campaign with Tests against Australia in Brisbane on August 10 and in Perth seven days later.