  Protostars likely accrete material at a highly time variable rate, however,
measurements of accretion variability from the youngest protostars are rare, as
they are still deeply embedded within their envelopes. Sub-mm/mm observations
can trace the thermal response of dust in the envelope to accretion luminosity
changes, allowing variations in the accretion rate to be quantified. In this
paper, we present contemporaneous sub-mm/mm light curves of variable protostars
in Serpens Main, as observed by the ALMA ACA, SMA, and JCMT. The most recent
outburst of EC 53 (V371 Ser), an $\sim 18$ month periodic variable, is
well-sampled in the SMA and JCMT observations. The SMA light curve of EC 53 is
observed to peak weeks earlier and exhibit a stronger amplitude than at the
JCMT. Stochastic variations in the ACA observations are detected for SMM 10 IR
with a factor $\sim 2$ greater amplitude than as seen by the JCMT. We develop a
toy model of the envelope response to accretion outbursts to show EC 53's light
curves are plausibly explained by the delay associated with the light travel
time across the envelope and the additional dilution of the JCMT response by
the incorporation of cold envelope material in the beam. The larger JCMT beam
can also wash out the response to rapid variations, which may be occurring for
SMM 10 IR. Our work thus provides a valuable proof of concept for the usage of
sub-mm/mm observations as a probe of both the underlying accretion luminosity
variations and the protostellar environment.
