How do I make a lofted cut with a 3 point slot?

As I am sure you can tell I am new to solidworks.    I am trying to design a shock piston.  Ive got it 95 percent complete but the last 5 is killing me!  I am making a port where one end is square and the other end is a 3 point arch.  When i try to connect the 2 ends is says "it can not be created because it would produce self intersecting geometry."  Any help would be very very appreciated. THX

SolidworksSketching