DP1 = Basic Infantry Qualification,
DP2A = Now called IPSWQ = Infantry Platoon Support Weapons Qualification (Exactly what it sounds like),
DP3A = Your Sergeant's course,
DP3B = Your Warrant's course,
However the entire NCM/NCO infantry training system is currently undergoing a big "renovation" so this is all subject to change.
EDIT: Also, forgot to mention, yes, DP stands for "Developmental Period", The full name for DP1 is "Developmental Period 1 (Infantry)"