Auto populate a field in the event based on values from another field

Auto populate a field in the event based on values from another field

The scenario with me today is that I have a table which looks like this 

Field A
Field B
Field C
a1
b1
c1
a2
b2
c2

In the event section, I would like to just fill in Field A .. Field B and Field C should get populated based on mapping of Field A 

Example I fill in a1 and for Field B and C they should get auto populated as b1 and c1 respectively. 

How do I do it ?