Check the ticktes to make sure. The 3 days must all be used within a 7 day period of first use. The first time you go through a turnstile, the "clock" starts ticking. You then have 7 days to use all 3.
We had bought Universal tickets through our AAA office back in the fall of 2002...knowing that we were going in the summer of 2003. We were able to get 2 days for the price of $75 (kids admision price...even for adults). When we got to Universal we saw the ad for "buy 2 days get 3rd day free". After we had played all day at Universal Studios, my husband went to guest services and asked if we could upgrade for our 3rd day free and he was able to do that...with no fee! So we were able to do 3 days for the price of $75. We ended up going back one afternoon (after spending the day at Typhoon Lagoon) and redoing Spiderman and MIB and ET! Our special surprise was...we were able to do Shrek as well. When we had gone earlier in the week, Shrek was still closed, so we felt that we definitely got our money's worth!