Aim Assessing the application of three‐dimensional clinical attachment loss (3D‐CAL), 3D supporting bone loss (3D‐SBL), supracrestal tissue attachment (STA), and crown‐to‐root ratio (CRR) in evaluating the 2017 periodontitis classification. Materials and methods We analysed ninety single‐rooted human premolars with micro‐computed tomography. The amount of 3D‐SBL, linear radiographic bone loss (RBL), and CRR corresponding to various periodontitis stages as well as the statistical significance was investigated. Results From a 3D perspective, the premolars with a 21% of 3D‐SBL at 2.0 mm coronal root length (RL) and 15% RBL corresponded to the periodontitis stage I. Premolars with a 44% of 3D‐SBL at coronal 4.2‒4.4 mm RL and 33% RBL accorded with the periodontitis stage II. Excluding the consideration of STA, CRR = 5:6 and 4:3 were associated with the levels at 15% and 33% RBL, respectively. Conclusions A greater percentage of 3D‐CAL than that of 2D‐CAL is significant at evaluated levels. It is feasible to correlate the 3D‐SBL, 3D‐CAL, and STA parameters to evaluate the stages of periodontitis severity. However, the current use of RBL and CAL as applied for staging in the 2017 classification might be inconsistent with the evaluated premolar roots length, when STA dimensions are considered.