Question

three reasons for mass immigration today

Answer 1

Three reasons for mass immigration today are economic reasons - plenty of opportunity in more advanced countries
social reasons - people fleeing in fear of being persecuted in their own country and war reasons -war wreaking havoc in a region makes people leave that region. 

Answer 2

Reason 1) Opportunity
People migrate in search of new jobs and work.

Reason 2) Freedom From Persecution
People flee because they are being persecuted.

Reason 3) Conflict
People flee their country because of war and conflict.