Not seeing what you're doing makes it hard to diagnose, but I would guess that the problem is when you are cutting the strips and sewing them together initially. You don't see the problem until you cut it into three's but it's there.
When you are cutting your strips you have to be sure to be precise, both when you measure them and then when you make the actual cut. It's very easy to veer off slightly and end up with a strip that's not even. Make sure before you cut that you've got the fabric folded perfectly.
When you are sewing a series of strips, sew the first two together in one direction, then add the next strip by sewing in the other direction. This will help with the wonkiness.
When you added the horizontal sashing, did you measure the piece of sashing or did you just take a long piece, sew it on and then trim it? You should measure the length of the sashing so they are exactly the same, pin at either end and then in the center, and add more pins if necessary.
I hope this helps!
Lynda