DiagTest3Grp: Diagnostic test summary measures for three ordinal groups. The R package provides utilities to estimate and make inference on two summary measures for diagnostic tests when there are three ordinal groups —-volume under ROC surface (VUS) and the extended Youden index, assuming that diagnostic markers increase monotonically and stochastically with disease severity. Point estimates, variance, confidence interval and optimal cut-points both under the normal assumption and also the non-parametric method(s) will be provided for the summary measures. Statistical tests are implemented to compare multiple diagnostic tests and two diagnostic test. Sample size is calculated to estimate the summary measures within a user-specified margin of error for future study planning