What Debbie says is very good advice! My boy isn't too frazzled by squirrels, but a cat makes him loco. I use "Leave it" at the very moment he begins to focus on the cat. That is his only warning. Once he turns and focuses on me, I throw him a party! Praise! Treat! And then I make sure we do something else to keep him focused on me. I would not make him sit or any command that requires him to be still. I might heel in figure 8's , or toss a ball to him (NOT THROW IT!). Something to get him to engage in another activity that does not include any opportunity to chase the cat.
I do not use "Leave it" if I missed the opportunity and he has already begun the chase (if he is off leash). I can't reinforce the command once he has lost his mind. At that point it's really my fault.
I only use 'Come' if I want him to come directly to me and either sit in front of me or at the heel position. If I'm using 'come' I'll point my hand either in front of me, or sweep it to my side. 'Come' is a direct command not to be taken lightly.
'Here' is a softer command I'd give if I want my dog to refocus on where I'm at. Like if I'm in the bedroom and he isn't. I'll say 'Here!' and he'll come into the bedroom, but he doesn't have to come and sit or glue to me. If I'm in the pasture and I say 'Here' that tells my dog that he has gotten too far ahead and I want him to come back towards me. He is to change direction and head towards me, but he doesn't have to come directly to me like he would with 'Come'.
Hondo Von Dopplet L Bauernhof "Hondo"- GSD
Lilie's Tug McGraw "Tug" - Golden Retriever
Maggie - Mini Dachshund (Rescue)
Lonestar Pivo - Texas Blue Lacy
Ashe - Barn Cat
Katie / APHA