The above posters are right re: reasonable suspicion. I don't think reasonable suspicion is the right thing to be talking about here. Moreover, I would add that reasonable suspicion / Terry stops are supposed to only be to ensure the safety of the officers - i.e. to search for weapons if the cops have a reasonable suspicion that the subject might be armed, in an attempt to elevate it up to probable cause.
I think the best way to approach this problem is chronologically. I am assuming the police had probable cause to stop the car in the first place. So, assuming that, when the officer approached and the driver admitted that he had had 3 beers, the officer at that point had probable cause to suspect DUI / drinking and driving. Thus, under the automobile exception, he could have searched the car without consent right then, before doing the field sobriety test, based on the probable cause derived from the driver's admission. But, after doing the 3 tests, the probable cause clearly dissipated, though it may very well be that there was still enough probable cause to do the search anyway, still based upon the driver's admission.
At least that's the way I would answer it.