Are you using install media such as an USB or DVD?
DISM /Online /Enable-Feature /FeatureName:NetFx3 /All /LimitAccess /Source:f:\sources\sxs
The letter after /Source: should be the drive letter of your install media(a windows 8 or 8.1 ISO) in this case my USB
Also the commands are letter case sensitive and spacing is important so open Notepad copy & paste my command in, change drive letter to yours, save, for future use, copy & paste in an Admin command prompt.