  Dusty star-forming galaxies (DSFGs) at redshift z$\geq$1 are among the most
vigorously star-forming galaxies in the Universe. However, their dense
($\geq$10$^5$ cm$^{-3}$ ) gas phase - typically traced by HCN(1-0) - remains
almost entirely unexplored: only two DSFGs have been detected in HCN(1-0) to
date. We present results of a JVLA survey of the J=1-0 transition of HCN, HCO+,
and HNC(1-0) in six strongly lensed DSFGs at z = 2.5 - 3.3, effectively
doubling the number of DSFGs with deep observations of these lines. We detect
HCN(1-0) emission in one source (J1202+5354, 4.4$\sigma$), with a tentative
HCO+ (1-0) detection in another (J1609+6045, 3.3$\sigma$). Spectral stacking
yields strict upper limits on the HCN/FIR ($\leq$3.6$\times$10$^{-4}$) and
HCN/CO(1-0) ratios ($\leq$0.045). The inferred HCN/FIR ratios (a proxy for the
star-formation efficiency) are consistent with those in z$\sim$0 FIR-luminous
starbursts. However, the HCN/CO ratios - a proxy for the dense-gas fraction -
are a factor of a few lower than suggested by the two previous DSFG detections.
Our results imply that most DSFGs have low dense-gas fractions. A comparison
with Krumholz & Thompson (2007) models of star-forming galaxies indicates that
the bulk of gas in DSFGs is at lower densities ($\approx$10$^2$ cm$^{-3}$ ),
similar to "normal" star-forming galaxies, rather than ultraluminous
starbursts.
