We present a study using the Karl G. Jansky Very Large Array ( VLA ) of \co J = 1 - 0 emission in three strongly lensed submillimetre-selected galaxies ( SMM J16359 , SMM J14009 and SMM J02399 ) at z = 2.5 –2.9 . These galaxies span L _ { IR } = 10 ^ { 11 - 13 } L _ { \odot } , offering an opportunity to compare the interstellar medium of LIRGs and ULIRGs at high redshift . We estimate molecular gas masses in the range 2 - 40 \times 10 ^ { 9 } M _ { \odot } using a method that assumes canonical underlying brightness temperature ( T _ { b } ) ratios for star-forming and non-star-forming gas phases and a maximal star-formation efficiency . A more simplistic method – using X _ { CO } = 0.8 and the measured T _ { b } ratios – yields gas masses twice as high . In SMM J14009 we find L ^ { \prime } _ { CO 3 - 2 } / L ^ { \prime } _ { CO 1 - 0 } = 0.95 \pm 0.12 , indicative of warm , star-forming gas , possibly influenced by the central active galactic nucleus ( AGN ) . We set a gas mass limit of 3 \sigma < 6 \times 10 ^ { 8 } M _ { \odot } for the Lyman-break galaxy , A2218 # 384 , located in the same field as SMM J16359 at z = 2.515 . Finally , we use the rest-frame \sim 115-GHz free-free flux densities for SMM J14009 and SMM J02399 – measurements tied directly to the photoionisation rate of massive stars , and made possible by VLA ’ s bandwidth – to estimate star-formation rates ( SFRs ) of 400–600 \textrm { M } _ { \odot } yr ^ { -1 } , and to estimate the fraction of L _ { IR } due to AGN .