This to me is a new revelation. That is a lot of mass and yet:
The effects of gravitational waves are so small that you’d need to be extremely close to a merger to feel them.
How is mass converted to gravitational waves?
The mass equivalent of kinetic energy. Energy and mass are equivalent; if you know one, you know the other through E=mc2. In other words, perhaps what is being converted to gravitational waves is the kinetic energy of the system (not its mass, per se) but which, of course, can nevertheless be expressed in solar masses. I was reading elsewhere on the net someone who said a star the size of the sun, which is flying through space at 40% the speed of light is packing with it the equivalent of one solar mass worth of kinetic energy. So that's what is going on: The kinetic energy of these two merging black holes is equivalent to the mass energy of three of our suns. And that kinetic energy, not the mass, is what is being released as gravitational waves.
Now, I know your next question will probably be, "Yes, but, how does kinetic energy get converted into gravitational waves?" LOL, don't ask! This was hard enough to come up with. :-)
Remember, I'm not a physicist. Just a math major who took some physics. And not enough either. I wish I had taken a lot more.
I keep asking the same thing. I keep reading about math/physics equations regarding gravity that change over time. I don't believe any physicist has an answer other than on their white boards, which often times is just theory and either change according to new math or are discarded like String Theory.
I'm probably like a heretic in Newton's day. I don't believe theories without empirical evidence. Although I do love Sci-fi books, movies, and some TV.
Here's something you can nail me on: I don't believe time is another dimension, as in space/time continuum. I believe it to be a human construct.