On my character creation screen, it says I have 3 boosted characters available. When I go to create a character, it gives me a choice of either a Level 60 or Level 65.

 

Can my 3 boosted characters be used interchangeably for either option, meaning I can choose Level 65 for all 3, or is there a limit to how many of the 3 can be created at Level 65?

You can make them all whichever option you want.

You should be able to make it for whichever characters you want. At least thats been my experience anyways. Hope this helps friend.