If you have three artists and a flatter, the breakdown should be something like...
1 artist on pencils
1 artist on inks
Flatter does flat color placement
1 artist finalizes color rendering
Then your letterer does the dialogue and sound effects
You could also do...
1 artist pencils and inks characters
1 artist pencils and inks backgrounds
Flatter does flat color placement
1 artist finalizes color rendering
Then your letterer does the dialogue and sound effects
Either way, you already have enough artists to work on your project, you don't need to bring in anyone else. "Too many cooks in the kitchen spoil the soup," you know.