With the high point of “Minions” being Gru’s brief cameo at the end, “Despicable Me 3” offers the franchise a chance to get back to what works best: the adventures of the formerly despicable Gru. Then again, this is the third instalment of a trilogy, so, you know… When Gru (Steve Carrell) and Lucy (Kirsten Wiig) are fired from the Anti-Villain League for allowing 80s-obsessed Benjamin Bratt (Trey…