This paper presents Petri net software tool P3 that is developed for training purposes of the Architecture and organization of computers (AOC) course. The P3 has the following features: graphical modeling interface, interactive simulation by single and parallel (with previous conflict resolution) transition firing, two well-known Petri net analysis tools (Reachability tree, Matrix equations), as well as two new analysis tools (Firing graph, and Firing graph) developed for learning purposes. The special aspect of the P3 is the XML/XSLT-based support for model sharing with the following Petri net tools: DaNAMiCS, Renew, and Petri Net Kernel (PNK). This paper also gives overview of the AOC course, and compares students' outcomes in the AOC course when they used the P3, with the previous course outcome when the students did not use P3. Finally, the paper shows how teachers (i.e. we) and students perceived P3's features. (Contains 1 table and 9 figures.)