We quantify the morphological evolution of z \sim 0 massive galaxies ( M _ { * } / M _ { \odot } \sim 10 ^ { 11.2 \pm 0.3 } ) from z \sim 3 in the 5 CANDELS fields .
The progenitors are selected using abundance matching techniques to account for the mass growth .
The morphologies of massive galaxies strongly evolve from z \sim 3 .
At z < 1 , the population well matches the massive end of the Hubble sequence , with 30 \% of pure spheroids , 50 \% of galaxies with equally dominant disk and bulge components and 20 \% of disks .
At z \sim 2 - 3 however , there is a majority of irregular systems ( \sim 60 - 70 \% ) with still 30 \% of pure spheroids .

We then analyze the stellar populations , SFRs , gas fractions and structural properties for the different morphologies independently .
Our results suggest two distinct channels for the growth of bulges in massive galaxies .

Around \sim 30 - 40 \% were already bulges at z \sim 2.5 , with low average SFRs and gas-fractions ( 10 - 15 \% ) , high Sersic indices ( n > 3 - 4 ) and small effective radii ( R _ { e } \sim 1 kpc ) pointing towards an even earlier formation through gas-rich mergers or violent disk instabilities .
Between z \sim 2.5 and z \sim 0 , they rapidly increase their size by a factor of \sim 4 - 5 , become all passive and slightly increase their Sersic indices ( n \sim 5 ) but their global morphology remains unaltered .
The structural evolution is independent of the gas fractions , suggesting that it is driven by ex-situ events .

The remaining 60 \% experience a gradual morphological transformation , from clumpy disks to more regular bulge+disks systems , essentially happening at z > 1 .
It results in the growth of a significant bulge component ( n \sim 3 ) for 2 / 3 of the systems possibly through the migration of clumps while the remaining 1 / 3 keeps a rather small bulge ( n \sim 1.5 - 2 ) .
The transition phase between disturbed and relaxed systems and the emergence of the bulge is correlated with a decrease of the star formation activity and the gas fractions suggesting a morphological quenching process as a plausible mechanism for the formation of these bulges ( although the eventual impact of major mergers and a growing black hole in the bulge should also be considered ) .
The growth of the effective radii scales roughly with H ( z ) ^ { -1 } and it is therefore consistent with the expected growth of disks in galaxy haloes .