You have your best chances for "late" availability at SSR because it's (by far) the biggest of all the
DVC resorts...it's also the most likely to open up due to cancelations (wait list success)...generally, one-bedrooms are more available for longer than studios, though--
Having said that, March includes spring break week...one of the two highest-cost and busiest weeks of the year at Disney...AND, all of DVC is designed to be full all the time...so the later you wait to reserve, the more "issues" you may encounter in finding space...
But, having said that...last year we decided 10 days ahead of time to go to Disney for spring break...and w/phone calls spread over 3 days, wound up getting four consecutive nights in a one-bedroom near the Hospitality House at OKW...OKW was more available than SSR and studios also came open (we just preferred the one-bedroom for that particular trip, so when it was available we took it)...
So...in my 3-year DVC experience, I've found that between 5 and 3 months out, availability, even at SSR, can be difficult...but that the wait lists always worked. And within 6 weeks or so (we've done that several times) we always found an acceptable place to stay (at either SSR or OKW), by making several phone calls and using the wait list.
And the non-spring break weeks in March are likely to be easier than the spring break weeks...
Good luck!