The K3 visa was created in 2000 to address processing backlogs at U.S. Citizenship and Immigration Services (USCIS) service centers regarding family-based immigration visas.
The K3 visa is intended to be used by U.S. citizens who are sponsoring their foreign spouse for immigration.

The K3 visa is a "non-immigrant" visa, meaning that receipt of a K3 visa does not result in the spouse becoming a legal permanent resident immediately upon their arrival in the U.S. The K3 visa does allow the spouse to travel to the U.S. and live with their U.S. citizen spouse while they are waiting for their I-130 immediate relative immigration petition to process through. They can also bring their unmarried children who are under 21 along using the related "K4" visa.
The process for applying for K3 visas is significantly more complicated than the process of applying for a K1 visas. The first step is to file an immigration petition for the foreign spouse.

This will involve gathering substantial amounts of information about both the foreign spouse and the U.S. citizen spouse. Although it is not technically required for the spouse's children to receive a K4 visa, it is best to go ahead and file immigration petitions for them as well. If the children are not biologically the children of the U.S. citizen spouse, then it is necessary to analyze the facts of the marriage to ensure that the U.S. spouse can successfully sponsor them for immigration. In most cases, this will be possible, but the rules in this area can be complex.

After obtaining a K3 visa, the foreign spouse and their unmarried minor children, using the K4 visa can travel to the U.S. and live with their spouse for up to 2 years. During that time the USCIS will approve the immigration petition filed by the U.S. sponsor and/or the spouse will be able to apply for adjustment of status to become a legal permanent resident.

After you receive a formal USCIS receipt or equivalent proof that you have filed the immigration petition, you can then file a K3 visa petition with the USCIS National Benefits Center. Unlike other immigration petitions (including K1 visa petitions), the National Benefits Center processes all K3 visa applications in the United States, so your K3 visa petition will go there regardless of where you live in the U.S.

Effect of "IMBRA" on K3 visa applications:
The International Marriage Broker Regulation Act ("IMBRA") took effect on March 6, 2006. IMBRA is intended to address perceived problems with the use of K1 or K3 visas by petitioners who have a history of domestic or serious criminal violence in their past, and with petitioners who file multiple K1 petitions. IMBRA affects K3 visa petitioners by requiring them to disclose any past history of domestic violence or serious criminal violence.