  We present high-fidelity, 30 milliarcsecond (200-pc) resolution ALMA
rest-frame 240 $\mu$m observations of cold dust emission in three typical
main-sequence star-forming galaxies (SFGs) at $z \sim 3$ in the Hubble
Ultra-Deep Field (HUDF). The cold dust is distributed within the smooth
disk-like central regions of star formation $1 - 3$ kpc in diameter, despite
their complex and disturbed rest-frame UV and optical morphologies. No dust
substructures or clumps are seen down to $\simeq 1- 3$ $M_\odot$yr$^{-1}$
(1$\sigma$) per 200-pc beam. No dust emission is observed at the locations of
UV-emitting clumps, which lie $\simeq 2-10$ kpc from the bulk of star
formation. Clumpy substructures can contribute no more than $1-7$% of the total
star formation in these galaxies (3$\sigma$ upper limits). The lack of
star-forming substructures in our HUDF galaxies is to be contrasted with the
multiple substructures characteristic of submillimeter-selected galaxies (SMGs)
at the same cosmic epoch, particularly the far-IR-bright SMGs with similarly
high-fidelity ALMA observations of Hodge et al. (2019). Individual star-forming
substructures in these SMGs contain $\sim10-30$% of their total star formation.
A substructure in these SMGs is often comparably bright in the far-infrared as
(or in some cases brighter than) our typical SFGs, suggesting that these SMGs
originate from a class of disruptive event involving multiple objects at the
scale of our HUDF galaxies. The scale of the disruptive event found in our
main-sequence SFGs, characterized by the lack of star-forming substructures at
our resolution and sensitivity, could be less violent, e.g., gas-rich disk
instability or minor mergers.
