The mandatory minimum depends significantly on whether the charge was prosecuted at the state or federal level, and in your boyfriend's case it sounds like the state handled it, which worked in his favor.
Under federal law, 18 U.S.C. § 922(g), a felon in possession of a firearm carries a mandatory minimum of five years with a maximum of ten. Federal prosecutors pursue these cases aggressively and there is very little flexibility at sentencing when the mandatory minimum applies. If the federal government had picked this up, a three-year sentence with parole eligibility at 18 months would not have been on the table.
The fact that he received three years with parole eligibility at 18 months tells you this was handled as a state matter. State felon in possession statutes vary widely. Some states have mandatory minimums of one to two years, others have more flexible sentencing ranges that allow judges to consider the specific circumstances of the offense. His judge apparently had enough discretion to arrive at three years, and the parole provision built into the sentence gives him a realistic shot at getting out at the halfway point.
At 18 months he goes before the parole board, and how that hearing goes depends on his institutional record between now and then. Clean conduct, program participation, and a solid reentry plan are what the board wants to see. A first appearance at the earliest eligible date with a strong record gives him the best shot at walking out at the 18-month mark rather than serving closer to the full three years.
Encourage him to treat every day between now and that hearing as an opportunity to build the case for release.