Here's what I do - I figure you're trying to link the Em back to the C to start the progression again, right? So, I use my ring finger exclusively, and start by hitting the open low E string, then the same string, second fret (F#), and third fret (G). Then, quickly the same thing on the A string, hitting it open, hammering on to the second fret ( B ), and sliding up to the third ( C ). Using my ring finger leaves my other fingers available so that by the time I've finished this little run, I have them in place to hit out the C chord. E----------------0 B----------------1 G--------------