Being Added as an AU

I Know this has probably been asked many times, but with AU everyone’s situation is unique and different. I am wanting to be added as a AU to my mother’s account to boost my fico. We are in the mist of trying to decide which card as I do not have a mixed of cards in my credit report

 

Card A: Has a 10,550 limit with a balance of 2,400 and is 15+years old

 

Card B: Has 1,500 Limit has 0 Balance (we plan to purchase something for $20 on it) and it roughly 3-4 years old

 

Card C: Has 1,000 limit, 0 balance (we plan to do a small purchase here also) and is roughly 3-4 years old also

 

I have 4 neg tradelines that are slated to fall off next month and my oldest current account was opened in 1995 (AU).

 

My Fico Credit scores are (Pulled by a lender)

 

636  EFX

567 TU (As you can see they do not like me:smileysad:)

601 XPN (FAKO score said it was 645)

 

 

Good news is most of my Transunion and Experian things are falling off next month and I have done PFD with the the low balance neg items.

 

Which card(s) would help me the most to get to 620 mid score.

 

All of my own positive Revolving TL have no lates and UTI is below 10% (Just paid them down from 30% UTI.

 

Any suggestion would be greatly appreciated.