Constat...saevitum esse:  one 'impersonal' construction followed by another.

With impersonals  the subject is abstract,  'it' rather than a specific person.

Constat means, 'it is agreed (that)', followed by indirect statement (acc. + inf.)

saevitum est  would be impersonal passive, lit. 'it was savaged' = savagery was committed. Infinitive --> saevitum esse.

The basic structure of the first 3  sections is then a sequence of infinitive phrases:

Constat

                   saevitum esse in ceteros Troianos

                   Achivos abstinuisse omne ius belli duobus (Troianis dat. of interest)

                   Antenorem venisse in Hadriatici sinum

                   Enetos Troianosque tenuisse eas terras

continuing in §4

                   Aenean profugum in Macedoniam venisse

                             inde in Siciliam delatum (esse)

                             ab Sicilia (delatum) ad Laurentem agrum tenuisse.