We present a study using the Karl G. Jansky Very Large Array (VLA) of ¹²CO J = 1−0 emission in three strongly lensed submillimetre-selected galaxies (SMMJ16359, SMMJ14009 and SMMJ02399) at z = 2.5–2.9. These galaxies span L_IR = 10¹¹–10¹³L_⊙, offering an opportunity to compare the interstellar medium of luminous infrared galaxies and ultraluminous infrared galaxies at high redshift. We estimate molecular gas masses in the range of 2–40 × 10⁹M_⊙ 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 SMMJ14009 we find , indicative of warm, star-forming gas, possibly influenced by the central active galactic nucleus (AGN). We set a gas mass limit of 3σ < 6 × 10⁸M_⊙ for the Lyman break galaxy, A2218 #384, located in the same field as SMMJ16359 at z = 2.515. Finally, we use the rest-frame ∼115GHz free–free flux densities for SMMJ14009 and SMMJ02399 – measurements tied directly to the photoionization rate of massive stars, and made possible by VLA's bandwidth – to estimate star formation rates (SFRs) of 400–600M_⊙yr⁻¹ and to estimate the fraction of L_IR due to AGN.