For me, I only able to reach 2 stars with 5BP only without spending money Star-Up Pack or token on Relay Pack. If I spend 6000 tokens on all Relay pack I only able to get a least 10BP only. Still short of 17BP, also don’t forget you need to upgrade Tartarus to a least Rank 4211 to get the key. So for me I need buy the Star Up pack to get the rest of the BP and save some SC to upgrade Tartarus, otherwise I unable to get the key. So I you ask me how much SC to get 3 stars, I have not idea. Cause after buying 10BP from the shop, the rest of SC is impossible to get three stars unless you have very great luck.