No amount of prosthetics or makeup could possibly make any of those 3 look remotely anything like Freddie Mercury.
And I'm pretty sure that it's much easier and cheaper to dub a singing voice than to transform an actor's appearance through CGI...
That why I said the looks are not the most important thing. But to counter your point, Jack Nicholson didn't look much like Jimmy Hoffa until they covered his face in prosthetics, and Anthony Hopkins or Frank Langela don't look much like Richard Nixon but still pulled off believable and compelling performances by getting the mannerisms and voice down. So it can work out either way. I just think it's far more important to not lip synch and to have a competent singer with a similar persona or a decent acting capability playing the role then it is to fixate on making him look identical to Mercury.
Nor did I say anything about using CGI, so I don't know where you're getting that from.
CGI was my comment on how improbable it is, IMO, to make any of those 3 look anything like Mercury even with a lot of prosthetics. IMO, the differences are greater than between Hopkins/Langella and Nixon or Jack Nicholson and Hoffa. It's not necessary for someone to look exactly like the person they're portraying, but there has to be at least some general resemblance (like Joaquin Phoenix/Johnny Cash). In these cases, you'd have to apply Star Trek level full face and body prostethics... and I'm talking Morn type of prosthetics rather than Quark or Worf.
And besides, the voice is even more of a problem, because you wouldn't be having an actor pretending to be Mercury, you'd be having Matt Bellamy or Justin Hawkins singing exactly the way Matt Bellamy or Justin Hawkins sing, only for some reason the movie will be telling you that it's actually Freddie Mercury, which would make no sense. Just like it never made sense to pretend that Diana Ross sang like Billie Holiday. No, that was Diana Ross singing and looking like Diana Ross. If you're not going to dub the voice, then it's better to find someone who is not a famous singer and who sounds approximately like the person they're meant to portray (like Sam Riley as Ian Curtis in
Control), then to cast another famous singer whose voice is very recognizable. Now, I must say I haven't heard Leto's music and all I know of his singing is what I heard in "My So-Called Life" (and OK, I don't think he's exactly that famous as a musician), but AFAIR he sounds nothing at all like Mercury. Neither do the other two.
None of those dudes can sing like Freddy. Not at all. Especially The Darkness guy with his nasally falsetto.
I adore Muse and I think Matt Bellamy is an amazing singer, but he doesn't sound like Freddie Mercury, he sounds like Matt Bellamy. Not to mention that he looks absolutely nothing like Mercury, either.