What would be the likely sentence for someone charged with 3 class 5 degree felonies?

Something doesn't sound right with what you're saying. Possession of an uncontrolled (but still prescription) substance is not a felony under South Dakota law. Possession of Schedule I or II controlled substance is a Class 5 felony punishable by a MAXIMUM of 5 years in the state penitentiary and/or a $10,000 fine for each count. However, it is also a presumptive probation offense, meaning that absent aggravating factors that pose a significant risk to the public, the judge should (keyword: should) suspended the prison time and place him on probation if convicted. No one can really tell you what the likely sentence would be without much more information about the facts and circumstances of the case, and if those aggravating factors exist. It would also depend upon if he has a basis to challenge any of the evidence against him. If your son does not already have an attorney representing him, he needs to get one ASAP. If he cannot afford an attorney, he needs to apply for a court-appointed attorney.