I am not sure how many people make it to the 3rd round of interviews. I made it to almost every 3rd round for positions that I applied for but not everyone gave me an on-site interview.
When you are invited for an on-site interview, you are a top 5 or top 4 candidate for the position. Some companies will decide to continue on-site interviews if they feel that none of their top candidates had strong interviews or because their top candidate chose another position.
Many positions will be filled this week, as I know of three people who have accepted positions. I will not be going to BMS but I know that two of their positions have been filled, one in med info/med strat and another in strategic analytics.