hello, on my non immigrant multiple entry visa, the utilsed date is 17th may 2012, am i right in assuming that if i do a border run on the 15th or 16 th i will get a final 3 months to see me through to 13th august. Any confirmation would be appreciated. thanks

Yes you are right - as long as you return prior to expiration of visa you will get a new 90 day stay.