Spurs will fill it, as they showed in CL. Massive capacity allows for cheaper pricing.
I suspect it won't be hard to get hold of tickets though, and to prevent the risk of the bigger teams with big away followings being scattered everywhere in the home end, they may opt to give more than 3k if required. Even if they give some teams between 5k-8k, still over 80000 available for Spurs fans which is more than Old Trafford