How to combine multiple entities into one with common attributes?

Currently, I need to combine multiple entities into the final entity, for example, entity 1 has 3 attributes “A, B, C” while entity 2 has “C, D, E” and entity 3 has “D, F, G”, in what way can I merge them into one new created entity? It would be best if someone could list out the steps as I am a beginner starting from scratch.  
1 answers

All the attributes are needed as the final one should have all attributes like"A, B, C, D, E, F, G". Moreover, the number of records inside these three entities is different, for example, 10 records in Entity 1, but 15 records in Entity 2, while Entity 3 has 12. Thus I need to merge them with the use of the key attributes but I still do not know how to achieve it.