You could do 2 or 3 filters in parallel. They will need changing a little more often than they would if you only had 3 ppm.
Your other option is to get an automatic system (
http://www.waterfilters.net/Iron-Whole-House-Filter-W-410-CFA_p_0-496.html), which will easily handle your amount of iron.
Thank you,
WaterFilters.NET