About waking at 3:00 PM
Three is the end of the afternoon dip and the point where alertness begins climbing again. It is also school pickup for a large part of the world, which makes it one of the more-set afternoon alarms.
Leave the tab open
A browser alarm can only ring while its page is open. It does not have to be the tab you are looking at, and on a desktop it does not have to be the window in front — but closing the tab or quitting the browser stops it. For anything you genuinely cannot miss, set a second alarm on your phone as a backup. That is true of every browser-based alarm clock, not just this one.