As far as I noticed, the Holy Spirit does not appear in Paradise Lost. When the Persons of the Trinity appear, it is only God the Father and God the Son. The reason for that is obvious enough to Bible readers: the more mysterious nature of the Holy Spirit. It is simply impossible to depict the Holy Spirit as a character in a literary work, because unlike with the Father or the Son, the Bible does not give us the kind of information needed to form a literary characterization. Milton must have conceded this, and avoided trying.
Milton posits that the pagan gods or idolatrous concepts described in the Old Testament--such as Beelzebub (Baal; "lord of the flies/dung heap"), Moloch (fire god requiring child sacrifice), mammon (money, materialism), and Belial--are inspired by actual individual demons corresponding to those characteristics described in the OT. That could be true, but not necessarily. The idols in the Bible could be inspired by demons other than Satan, or they could all be inspired by Satan himself. There is no way to be sure.