Is This 1918 British Penny, Counterfeit Or Real? It Weighs Three Point 86 Grams

You haven't given us much to go on here! What date penny are we talking about? They have been produced in this country for over 1200 years in many different forms.
Decimal pennies should be 3.56g. The large pre-decimal penny 1860 to 1970 should be around 9.45g. Prior to that there are many possible weights in copper or silver.
Modern coins are struck in collars, so the possibility of weight being squished out the sides during pressing is not realistic. Underweight coins are usually because they have been struck on the wrong or damaged planchet. Fake is far more likely - pictures would help. If you can't do that, diameter and thickness, along with date will get us closer.

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