All 3 of those signatures on the letter to POTUS are from the FREEDOM Caucus.
>FREEDOM Caucus
History
The origins of the caucus lie at the mid-January 2015 Republican congressional retreat in Hershey, Pennsylvania. Nine conservative active Republican members of the House began planning a new Congressional caucus separate from the Republican Study Committee and apart from the House Republican Conference. The group ultimately became the nine founding members and the first board of directors for the new caucus consisting of Republican Representatives Scott Garrett of New Jersey, Jim Jordan of Ohio, John Fleming of Louisiana, Matt Salmon of Arizona, Justin Amash of Michigan, Raúl Labrador of Idaho, Mick Mulvaney of South Carolina, Ron DeSantis of Florida and Mark Meadows of North Carolina.
https://en.wikipedia.org/wiki/Freedom_Caucus