So we are currently running a muthic campaign and I'm a level 7 sorceeer 3 deominiac and I picked up legendary item as a mythic path ability. I am putting it on a glabrezu claw that I currently have that is already a minor artifact. My question is does it become a major artifact or do I still have to take legendary item 3 times. Also would I then get the number of powers that major artifact gets under legendary item.
I wouldn't think you'd be able to select an artifact as a legendary item.
When a magic item ascends to legendary status, it gains the suite of base legendary abilities and can have up to three additional legendary abilities. By selecting the legendary item universal path ability more than once, a mythic creature can transform a normal legendary item into either a minor artifact (if the character chooses it a second time) or a major artifact (if the character chooses it a third time).
Artifacts can be Legendary Items of their own accord, but I don't think a pre-existing artifact would be a suitable selection.
That said, even if you did, you would still be limited to the progression detailed above.