Is via cap-space. Forget trades. We won't have the assets.
Even if we somehow kept Felton/Gallo/Mozgov out of the deal, a Felton/Gallo/Mozgov package is not going to net us one of the top 2 PG's in the league. People overestimate Gallo's trade value cause he's our own homegrown product, but next season, he'll be an impending RFA just like Chandler is this year, and his stats will likely go down even further with Melo taking away his minutes and shot attempts. Moreover, that 2012 pick we conveyed to Houston will STILL be c*ckblocking us from trading any picks in a CP3/Deron trade. Simply put, we're not going to have the assets to pull a trade off, even if we manage to preserve our assets from a Melo trade.
So where does that leave us? Freeing up cap space. It's too speculative to determine how much we'd need, because we simply don't know what the new CBA will entail. Some reports say that there could be a hard cap as high as the current luxury tax level ($70M), which would obviously allow us to sign CP3 or Deron with ease. Furthermore, if there is a hard-cap, the league may allow us to renegotiate all current contracts, as they did with the NHL, and we could possibly request Amar'e to lower his max in exchange for an extra year. Or on the other end of the spectrum, a franchise tag could completely bar us from acquiring either of them.
But for these calculations, I'm going to assume two things -- that the salary cap goes up to $60M by 2012, and CP3/Deron's max will be $16M. Technically it's $16.5 under current CBA rules, but for simplicity's sake (and the hope that they'd take a slight paycut or the CBA reduces the max), let's just say we need to clear $16M in cap space.
Here's what we need to happen to clear enough room:
1) Melo needs to forego any raises in 2011 and 2012, so that his salary in 2012 is only $17.2M. Hard sell, yes, but considering what we're reportedly giving up for him, he better play ball. He only loses roughly $5M over the course of his contract, and Amar'e can offer to pay for all team dinners and champagne at clubs. 
2) We will have to choose between Fields or Mozgov, unless the cap goes up to $63M or higher.
2a) Mozgov is on the books for $3.1M in 2012, but only $1.4M is guaranteed. Still, if we could squeeze him in, we'd be better off, as buying him out would only clear an additional $1M in cap space (cause we'd still be hit for his $1.4M salary, and the $500k roster charge that would result from losing another roster player)
2b) Fields is a RFA, but his cap hold while he remains unsigned is the league average salary (or the MLE), which can be estimated to be around $5.5M. To fit Fields into our 3-max plan, we have to convince him to sign a one-year $2-3M salary. This may seem far-fetched, but note that as a 2-year rookie, he can't get more than the league average salary (or the MLE) on the open market, thanks to the Gilbert Arenas rule. So we're essentially asking him to sacrifice $2-3M for one year, with the promise of a bigger pay-day once we have his bird rights the following summer.
3) Renounce Douglas and trade whatever player we've chosen with our 2011 pick
Resulting salaries:
2012 Salary -Moz -Fields -M/F
Amar'e $19.9M
Melo $17.2M
Fields $3.0M
Mozgov $3.1M ($1.4M)
7 Roster charges $3.5M ($4.0M) ($4.0M) ($4.5M)
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TOTAL $46.7M ($45.5M) ($44.2M) ($41.6M)
CAP SPACE $13.3M $14.5M $15.8M $18.4M
As you can see, it's really cutting it close, but if we wanted to renounce both Mozgov and Fields and have zero supplementary parts (besides vet min signings), we could conceivably form a big 3. If we wanted to include one of Moz/Fields, we either have to hope the cap goes up to $61-62M, or CP3/Deron takes a LeBron-esque paycut. Keeping both Fields and Moz will require a cap of $63M.
Really, it all begins with Melo agreeing to take a paycut on his extension. If he wants his full max ($20M by 2012), there is no real way to fit in Mozgov OR Fields, and even then, we may still be shy of being able to offer the max, unless the cap goes significantly up.
CONCLUDING POINT IF TLDR: MELO NEEDS TO TAKE A PAYCUT