We analyze the characteristics and suppression of multiples generated in three dimensions. We derive expressions for the traveltime of peg-leg multiples generated by two dipping planes. Using this theory, we demonstrate that multiples generated in three dimensions may have traveltimes substantially different from 2-D multiples. For a case studied of 3 degrees cross-line dip, second-order water-bottom and peg-leg multiples arrive on the order of 10 ms earlier than they would in two dimensions. For fifth-order multiples, this difference is as high as 65 ms. These 2-D/3-D time differences grow to many hundreds of milliseconds for 10 degrees of cross-line dip. We confirm the presence of the cross-line dip effect in field data containing salt-related multiples. Using dips estimated from a 3-D interpretation of a top-salt horizon, we further demonstrate that the theory can accurately predict traveltimes for such multiples. Finally, based on the theory, we present an algorithm for suppressing 3-D multiples when cross-line dip can be estimated, validating the algorithm using model and field data.