50bmg wrote:So, gravitons are carrying away the energy/mass?
I wouldn't say that, because gravitons are purely hypothetical particles. An elegant quantum treatment of gravity largely requires them, but we don't have such a treatment yet. So the safe answer is that gravitational radiation is the form the energy is in.
In a nuclear reaction where mass is lost in the form of radiated energy - are particles "turned into" photons as they radiate away, and in this way mass/energy is lost?
Mass is converted to energy, and if that energy is electromagnetic, photons are its carriers.
Then, in the case of the merging black holes losing mass/energy -
Where did this mass/energy come from that was lost (3 solar masses in this case)? Are there now fewer particles (three solar masses worth of particles) in the converged black hole than there were in the two black holes?
The merged black hole has less mass than the sum of the progenitors. We don't know if black holes even have "particles" inside, so best not to go there. Somehow, mass escaped. Presumably that's something that happened right at the moment of merger, where the event horizon is very complex.
And, if so, did these particles "turn into" gravitons and radiate away mass/energy?
Possibly, assuming that gravitons exist. As noted above, what we can reliably say is that the mass (not "particles") was converted to energy (via mass-energy equivalence) in the form of gravitational radiation, the carrier of which is uncertain.