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.
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