I think you are under-estimating the amount of time that will be taken up by fetching errant balls, arguing discussing rules/formats/playing order/etc. Squeezing into a 3 hour timeframe is overly ambitious imo - you need to have more contingency room..... create your structure to aim for two hours, and it will probably run for three.
If the first stage is a round robin, there really is no need for BO3 games.... BO1 is sufficient. To alleviate the perceived negative impact of BO1, you could switch the games to 15 points per game with 3 serves per handover, as opposed to 11 points with 2 serves. When you reach the KO stage, you then switch to 11 point games with 2 serves, and change to BO3