The K-3 visa is available to the spouses of US citizens and the unmarried children under 21 of those spouses who are outside the US awaiting processing of their Green Card petitions. To qualify, the applicant need only have a US citizen spouse petition on his or her behalf.
The visa is of particular benefit to children over 18 but under 21, who are not the natural children of the petitioning spouse. This group is now able to enter the US on the K visa, which allows them to live and work in the US until their adjustment applications are approved.