Sup, writer here. How much of a grip do you have on each of these character's personalities? Can you "feel" how each one would react in a given situation? It's like how when you become good friends with someone, you start to be able to guess how'd they react to certain things. You have to achieve that level of understanding with your characters for them to become truly alive and consistent, but depending on what you're writing you don't always have to go that far.
An easy way to achieve this is to have a separate document where you plan these characters out. It doesn't have to be anything fancy. List the characters, several major traits (i.e. age, gender, their past, etc) and then a few words for what their personality is like. With that, you now have a guide for you to consult back to if you feel like you're losing your grip on who these characters are.
Finally, the best way to guarantee consistency is revisions. Fight the urge to get everything right in the first draft, that urge is the death knell of good writing. Just get the shit you think of down, walk away from the material for a bit, then revise however many times you need to. Read through the piece and edit as you go. As you read through you can keep track of the characters to see if they're truly consistent, along with any other problems you feel you need to address. Hope this helps!