Efficiency of a 3 element wing being unrealistically high

There’s no project link for us to have a look, but in case someone else stumbles upon this, here are some common reasons why this could happen:

  1. Wrong boundary conditions (wrong units, wrong velocity inlet direction, etc.)
  2. If lift and drag coefficients are way off, there could be something wrong with your definition of reference area and/or reference velocity (see here for more details)
  3. Poorly converged results (see here for more details)

Cheers