I'm going to keep this to names only in the interest of minimizing spoilers.

#Key
**#**: Only availiable as a continued romance if initiated in Mass Effect 2  
**^**: Same Sex Romance option  
**%**: Not a Squad Member  
**$**: Does not count towards 'Paramour' achievement  

**All listed relationships *except* for Diana Allers count for Paramour.**

#Male Shepard

**Ashley**    
**Liara**    
**Tali** #    
**Miranda** #%    
**Steve Cortez** ^%  
**Kaidan** ^  
**Jack** #%  
**Diana Allers** %$

#Female Shepard

**Kaidan**    
**Liara** ^  
**Garrus** #  
**Samantha Traynor** ^%  
**Diana Allers** ^%$