I'm not sure about the three month requirement. I always thought once you established a home and got your DL you were considered a resident of SC. SLED probably has their own requirements that are not listed. An out of state FFL doesn't care if you are a resident or not. All they see is a SC DL. Once they ship it to a SC FFL you have to meet SC requirements and fill out the 4473 to pick up the firearm.
Background checks are required on all dealer transaction in every state. Fed law! One exception in SC is if you have a SC CWP then you are exempt from background checks. One great benefit of having a CWP. There is no waiting period in SC. Just fill out a 4473, pay for the firearm and carry it home. For CWP class and others residency questions I'd recommend calling SLED and find out their requirements.
Concealed Weapons Permits, Security Guard, and Private Investigator Questions
803.896.7099
USAF Retired, CATM, SC CWP, NH NR CWP, NRA Benefactor
To preserve liberty, it is essential that the whole body of people always possess arms, and be taught alike, especially when young, how to use them... -- Richard Henry Lee, 1787