A three-reel slot has three reels, usually showing one or three rows. It is the original slot layout, used by Charles Fey's Liberty Bell and by most mechanical machines that followed.
With only three reels there are fewer combinations to count, so paytables are short and easy to read. Many have a single payline across the middle; others have five lines across three rows. Wins usually need three matching symbols, sometimes with a payout for one or two of a particular symbol, such as cherries.
Three-reel games online are usually styled as classic slots. The format says nothing about RTP or volatility, which vary by game and are shown in the paytable.