According to my obsessive Googling, these three representatives are Evangelical or Pentecostal. This makes some sense, given that neither category is listed separately in the map key. If they followed their state demographics, they'd be more likely to be Lutheran or Catholic, but just because they're Representatives doesn't mean they're representative in all ways.
North Dakota: Rep. Kevin Cramer
Cramer's background is Lutheran, "[b]ut he soon found his politics and philosophy to be too conservative for the church"; his religious affiliation is given by the National Journal (1 November 2012) as "Evangelical Christian". A comment on an article in the Great Plains Examiner (25 April 2012) says "Cramer is a very active member of New Song church in Bismarck". New Song Church is affiliated with the International Church of the Foursquare Gospel (ICFG), an evangelical Pentecostal denomination.
South Dakota: Rep. Kristi Noem
Some blogs (1, 2, 3, 4) report that Noem is a member of the Family Worship Center Foursquare Church (possibly dead link) in her hometown of Watertown, SD. This is also an ICFG church.
Nebraska, 3rd district: Rep. Adrian Smith
According to this 2006 page, Adrian Smith was affiliated with Calvary Memorial Church in Gering, NE. This church is part of the Evangelical Free Church of America. On 15 July 2009, Smith introduced its pastor, Rev. Gary Hashley, to be a guest chaplain of the US House. Later, on 14 March 2013, he did the same for Rev. Dr. Ezekiel Pipher, who is senior pastor of another EFCA church, Heartland Evangelical Free Church of Central City, NE. Buzzfeed (see below) categorize him under EFC.
"Other Christian" generally
The list form of the Buzzfeed map says exactly who they categorized as "Other Christian", with several subcategories. In summary:
- Assemblies of God (2)
- Christian (11, including Cramer and Noem)
- Christian Reformed (1)
- Christian Scientist (2)
- Congregationalist (4)
- Churches of Christ (2)
- Church of God (1)
- Disciples of Christ (1)
- Eastern Orthodox (5)
- Evangelical Free Church (2, including Smith)
- Fellowship of Evangelical Churches (1)
- Nazarene (1)
- Non-denominational Christian (3)
- Pentecostal (1)
- Protestant (10)
- Quaker (1)
- Seventh-Day Adventist (2)
- United Brethren (2)
- Unspecified Christian (5)
Aside from the three already mentioned, the other "Other" representative in the same general area is Michelle Bachmann of Minnesota's 6th district. Buzzfeed classifies her as generic Christian. Reportedly, she attends Eagle Brook Church, an evangelical megachurch.