At the risk of pointing out the obvious... Trump didn't have to bring Michael Flynn, Carter Page, or Paul Manafort onto his campaign. The most generous possible interpretation: Trump has terrible judgment, and carelessly put himself at risk with these voluntary associations.
Sticking with the generous interpretation: Flynn was one of the only people with an impressive resume to support Trump early besides Jeff Sessions. Manafort was shady, but at least had political success as an operative in other countries. As for Page... ¯\_(ツ)_/¯