This article compares the journal coverage of Web of Science, Scopus, and Dimensions, aiming to describe, understand, and visualize the differences among them. The most recent master journal lists of the three databases are used for analysis. The results show that the three databases have significantly different journal coverage, with Web of Science being the most selective and Dimensions the most exhaustive. About 99.11% and 96.61% of the journals indexed in Web of Science are also indexed in Scopus and Dimensions, respectively. Scopus has 96.42% of its indexed journals also covered by Dimensions. Dimensions has the most exhaustive journal coverage, with 82.22% more journals than Web of Science and 48.17% more journals than Scopus. The article also analyzed the research outputs for 20 selected countries for the 2010-2018 period, as indexed in the three databases, and identified database-induced variations in research output volume, rank, global share, and subject area composition for different countries. It is found that there are clearly visible variations in the research output from different countries in the three databases, along with differential coverage of different subject areas by the three databases. The analytical study provides an informative and practically useful picture of the journal coverage of Web of Science, Scopus, and Dimensions databases.This article compares the journal coverage of Web of Science, Scopus, and Dimensions, aiming to describe, understand, and visualize the differences among them. The most recent master journal lists of the three databases are used for analysis. The results show that the three databases have significantly different journal coverage, with Web of Science being the most selective and Dimensions the most exhaustive. About 99.11% and 96.61% of the journals indexed in Web of Science are also indexed in Scopus and Dimensions, respectively. Scopus has 96.42% of its indexed journals also covered by Dimensions. Dimensions has the most exhaustive journal coverage, with 82.22% more journals than Web of Science and 48.17% more journals than Scopus. The article also analyzed the research outputs for 20 selected countries for the 2010-2018 period, as indexed in the three databases, and identified database-induced variations in research output volume, rank, global share, and subject area composition for different countries. It is found that there are clearly visible variations in the research output from different countries in the three databases, along with differential coverage of different subject areas by the three databases. The analytical study provides an informative and practically useful picture of the journal coverage of Web of Science, Scopus, and Dimensions databases.