Background: Misconceptions are major obstacles to learning physics, and the concepts of heat and temperature are some of the common misconceptions that are encountered in daily life. Therefore, it is important to develop valid and reliable tools to determine students' misconceptions about basic thermodynamics concepts. Three-tier tests are effective assessment tools to determine misconceptions in physics. Although a limited number of three-tier tests about heat and temperature are discussed in the literature, no reports discuss three-tier tests that simultaneously consider heat, temperature and internal energy. Purpose: The aim of this study is to develop a valid and reliable three-tier test to determine students' misconceptions about heat, temperature and internal energy. Sample: The sample consists of 462 11th-grade Anatolian high school students. Of the participants, 46.8% were female and 53.2% were male. Design and methods: This research takes the form of a survey study. Initially, a multiple-choice test was developed. To each multiple-choice question was added one open-ended question asking the students to explain their answers. This test was then administered to 259 high school students and the data were analyzed both quantitatively and qualitatively. The students' answers for each open-ended question were analyzed and used to create the choices for the second-tier questions of the test. Depending on those results, a three-tier Heat, Temperature and Internal Energy Diagnostic Test (HTIEDT) was developed by adding a second-tier and certainty response index to each item. This three-tier test was administered to the sample of 462 high school students. Results: The Cronbach alpha reliability for the test was estimated for correct and misconception scores as 0.75 and 0.68, respectively. The results of the study suggested that HTIEDT could be used as a valid and reliable test in determining misconceptions about heat, temperature and internal energy concepts.