The number of artifacts you control is checked only once: when Concussive Bolt resolves. If you don't control three or more artifacts at that time, creatures will be able to block that turn, even if you gain control of more artifacts later in the turn.

If the player is an illegal target when Concussive Bolt resolves, the spell will be countered. None of its effects will happen, even if you control three or more artifacts.

If you controlled three or more artifacts as Concussive Bolt resolved, the targeted player won't be able to block with any creatures this turn. That is, the set of affected creatures isn't locked in as the spell resolves.