You could do this by modifying the Mango/web/modules/mangoUI/web/mangoUi.js file. The string Mango v3 only appears in the relevant place (you'll need to address the use of titleSuffix elsewhere in the file, probably).
The <title> element is in Mango/web/modules/mangoUI/web/ui/index.html
Modifying those will not persist through an upgrade. While you could put them into Mango/overrides/web/modules/mangoUI/web[/ui] I would not advise it, since that could break other things in the UI upon update.
Edit: See this thread for a better way to do it: https://forum.infiniteautomation.com/topic/4457/cutomizing-the-tab-name