So, if you have a string like "3 MMT-Aug 2015", then I wonder if you additionally have values such as "YTD-Aug 2015" and how that data is structured. That might help in knowing some of the options you have.
If you can convert your strings to a date, then you'll have a lot of flexibility with Tableau's built-in date features. You could get dates with code like this:
DATEPARSE(REPLACE([date string], "3 MMT-", ""), "MMM yyyy")
Hope this helps!