To speak to this, when I was applying for HPSP last cycle, I contacted Navy and missed the cutoff for the 4-year. This was fine as I was most interested in pursuing the 3-year. However, when I talked to the recruiter he said that the 3-years go for the roll-overs from the 4-year (alternates, etc). This wasn't just my recruiter either, as it was confirmed by many I know in the Navy.
TL;DR Navy 3-year doesn't really exist if you didn't apply for 4-year. Go another route if you want a 3-year.
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