Just use parens and do the division:
(If(dd1.Selected.Value="yes",1,0) + If(dd2.Selected.Value="yes",1,0) + If(dd3.Selected.Value="yes",1,0))/3
Or, to get it formatted as a percent, you can use the "text" function:
Text((If(dd1.Selected.Value="yes",1,0) + If(dd2.Selected.Value="yes",1,0) + If(dd3.Selected.Value="yes",1,0))/3*100,"0%")