Basic Explanation: 

Portals created: 

Over-world A, B, and C 

Nether 1, 2, and 3

If you take Over-world portals A, B and C you appear in Nether portal 1 

if you take Nether portals 1, 2,  or 3 you appear back at Over-world Portal A 

This has been tested in realms and in a normal survival in multiple locations and different times of day. you always end up at the original portals made.