Bluetooth uses what are called piconets. Each piconet is made up of at least one master and as many slaves as the bands will allow. No bluetooth device can be the master of more than one piconet but a bluetooth can be the slave of many.
This means that what you are looking for can only be done where each headset is the Master of its own piconet and the device delivering audio is the slave of them all. I've never tried it but I dont think the built in bluetooth for the RPi3 will handle this correctly, unless someone has come out with an epic package for that.
The only thing i can think of that would work is a splitter, something designed specifically to act as the slave of multiple BT devices. Like this one from Walmart or these ones from Amazon. Take care: a lot of them however, only work for two piconets, meaning two headsets. I don't know of any that do more, but id love to see one.
The problem comes from managing the frequency hopping that must be done to maintain paired connection. The more connections the more managements it takes to make sure the chirps don't overlap. And the master is the device that decides what clock to synchronize and what frequency schedule to maintain, further complicating things.
Haven't looked at these in a while, so shop around, maybe the tech got better and there are some out there that will do more than 2? But AFAIK its not as easy as it sounds like it should be.
On a side note, something I used to do a while back was get over the ear radio headphones that could pick up AM/FM frequencies. Then I set up a short range radio transmitter and used this to supply as many headphones as i desired for an outdoor movie night against a brick wall. It was a blast and worked like a charm. Bluetooth is awesome and if you want to stick with it that's fine, but it wasn't designed for this.
Just throwing it out there that technologies do exist that were designed exactly for this (think car radio). Many users, one source.