There are many important moments in the story, but the most crucial—called “story spotlights”—are shown on cards. These cards have the Planeswalker symbol in their text box; this symbol has no effect on gameplay. You can read more about these events in the official Magic fiction at http://www.mtgstory.com.
No player may take actions in a turn before Inventors' Fair's triggered ability checks to see if it should trigger. If you don't control three or more artifacts, it won't trigger.
If you control three artifacts as the ability resolves, you gain 1 life. The artifacts you control as the ability resolves don't have to be the same ones you controlled as it triggered. If you don't control three artifacts at that time, you won't gain life.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Jonas De Ro
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When using Inventors' Fair's activated ability, the number of artifacts you control is checked only as you activate it. It's not checked again as the ability resolves.
