How To Add 3 Days To The Existing Date

Bottom is my table structure. please some one can give me the sql code to add 3 days to the existing date.  

SemesterID int
SemesterLongID varchar
SemesterIdentifier char
SemesterIdentifierName varchar
SemesterName varchar
SemesterStart datetime
SemesterEnd datetime
SemesterNameAlt varchar
EnrollmentDeadline datetime
RegistrationStart datetime
RegistrationEnd datetime
LengthInWeeks varchar
BulletinStartDate datetime
BulletinEndDate datetime
ClassroomStartDate datetime
ClassroomEndDate datetime
FacClassStartDate datetime
FacClassEndDate datetime
PlanningStart datetime
PlanningEnd datetime
GoToTeach datetime
TOButAppears datetime
TOButDisappears datetime
Signature datetime
TOButDue datetime
BookListDue datetime
ProfAccess datetime
ExamDue datetime
SCGDue datetime
GradesDue datetime
ExtGradesDue datetime
TOExt datetime
SessionPlanning datetime
TODue datetime
SCGStart datetime
TOExpire datetime
SRPTOCDue datetime
SRPCopyDue datetime
SRPCCCFormDue datetime
TOID int
SemesterIdentifierID int
Code varchar
TitleIVDescription varchar
CreatedBy varchar
ModifiedBy varchar
DateCreated datetime
DateModified datetime