Potassium permanganate to analyze iron compounds
The Standard Methods for the Examination of Water and Wastewater 3500-Fe is a method for determining total $\ce{Fe}$, $\ce{Fe^{2+}}$ and $\ce{Fe^{3+}}$ in water using a colorimetric detection technique.  As in your titrimetric method, this method can only directly measure $\ce{Fe^{2+}}$.  In order to measure $\ce{Fe^{3+}}$ (the only other reasonably stable oxidation state of $\ce{Fe}$) you must first reduce the $\ce{Fe^{3+}}$ to $\ce{Fe^{2+}}$.  I believe you could use the same reduction technique to prepare a solution of $\ce{Fe^{3+}}$ for your titration method.  

The procedure sited above states that:  


  "[the $\ce{Fe}$ in the sample is] reduced to the ferrous state by boiling
  with acid and hydroxylamine."


The hydroxylamine solution is prepared by dissolving 10 g $\ce{NH2OH⋅HCl}$ in 100 mL water.  50 mL of the water sample is acidified with 2.0 mL of concentrated $\ce{HCl}$, and 1 mL of the $\ce{NH2OH}$ solution is added to the acidified water sample.  

As $\ce{Fe^{3+}}$ species are frequently insoluble, the procedure described above calls for boiling the solution down to 15-20 mL to get all of the iron both dissolved and reduced before bringing the solution back up to 50 mL for analysis.  If the solution were prepared from a soluble $\ce{Fe^{3+}}$ salt like $\ce{FeCl3}$ for example, simply boiling the solution for 5 minute or so would likely be sufficient.  

So, I suspect you could measure the $\ce{Fe^{3+}}$ in your solution using your titration method if you first reduced the $\ce{Fe^{3+}}$ to $\ce{Fe^{2+}}$ using the hydroxylamine reduction described above.