I spent TWENTY TIMES more upgrading those 3 mythicals…
Heres my idea on this.
Persay you have a mythical … brightroar (its bad lmao ) at level 50.
If you make a new mythical ,you can fuse ALL 50 levels in the mythical for the price of 500K (the price needed to max an myth)
Basically for short : You have a bad myth item fully fused , and you make a myth item you need , you can fuse the bad myth into your new one and max it for the price needed.