The Hubble sequence is a common classification scheme for the structure of galaxies .
Despite the tremendous usefulness of this diagnostic , we still do not fully understand when , where , and how this morphological ordering was put in place .
Here , we investigate the morphological evolution of a sample of 22 high redshift ( z \geq 3 ) galaxies extracted from the Argo simulation .
Argo is a cosmological zoom-in simulation of a group-sized halo and its environment .
It adopts the same high resolution ( \sim 10 ^ { 4 } M _ { \odot } , \sim 100 pc ) and sub-grid physical model that was used in the Eris simulation but probes a sub-volume almost ten times bigger with as many as 45 million gas and star particles in the zoom-in region .
Argo follows the early assembly of galaxies with a broad range of stellar masses ( \log M _ { \star } / { M } _ { \odot } \sim 8 - 11 at z \simeq 3 ) , while resolving properly their structural properties .
We recover a diversity of morphologies , including late-type/irregular disc galaxies with flat rotation curves , spheroid dominated early-type discs , and a massive elliptical galaxy , already established at z \sim 3 .
We identify major mergers as the main trigger for the formation of bulges and the steepening of the circular velocity curves .
Minor mergers and non-axisymmetric perturbations ( stellar bars ) drive the bulge growth in some cases .
The specific angular momenta of the simulated disc components fairly match the values inferred from nearby galaxies of similar M _ { \star } once the expected redshift evolution of disc sizes is accounted for .
We conclude that morphological transformations of high redshift galaxies of intermediate mass are likely triggered by processes similar to those at low redshift and result in an early build-up of the Hubble sequence .