I can't find Function plugin in construct 3. But Documentation says This means that it is not downloadable via these pages as it's already included within Construct 3 itself.
It's available in runtime 3 (advanced settings to change from 2). Go to the bottom of the event sheet, then right click + add function.
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The functions feature was redesigned and made built-in over a year ago. See the blog posts Construct 3's new redesigned functions and More about Construct's new functions.