Mandatory is a mandate imposed upon you.
Obligation is something you take upon yourself.
Compulsory: dunno

Oh my word. I'm impressed.
Compulsory is something you can generally ignore.
Compulsory basic training (CBT) was introduced in 1990 to help reduce the very high accident rate among inexperienced motorcyclists. CBT must be completed before a learner moped or motorcycle rider is allowed to ride on the road with L-plates or D-plates in Wales
Mandatory is a mandate imposed upon you.
Obligation is something you take upon yourself.
Compulsory: dunno