Beta Update:
The Season 1 like costume bonus only applies to Legendary Season 3 hero costume.
As I recall all other Season 3 costumes have lower costume bonus similar to Season 2 costume bonus.
(There are no Rare S3 costumes in Beta so I could not check it)
We have seen it earlier, but we thought that this is a bug, but Staff has just confirmed that it different by design.
I have added a poll for collecting feedback about this to the OP.