Fluoroquinolones such as ofloxacin are promising drugs to treat drug-resistant tuberculosis (TB) and have been proposed for shortening the treatment of TB. The objectives were to study the synergistic effect of the combinations of three drugs and to evaluate the in vitro interactions of the following combinations against Mycobacterium tuberculosis: A) isoniazid, rifampicin, and ethambutol and B) ofloxacin, rifampicin, and ethambutol using an adaptation of the two-dimensional chequerboard assay. A total of 12 isolates resistant to isoniazid or to isoniazid-streptomycin and 11 drug-susceptible isolates were tested. The fractional inhibitory concentration (FICI) was calculated as follows: FICI = FIC_A + FICB + FIC_C = A/MIC_A + B/MIC_B + C/MIC_C where A, B and C were the MICs of each antibiotic in combination and MIC_A, MIC_B and MIC_C were the individual MICs. The FICI was interpreted as synergism when the value was ≤0.75. In combination A, 11 drug-susceptible isolates decreased the individual MIC one to three dilutions, showing indifferent activity in 81.8% (FICI = 0.88–1.6) and synergistic activity in 18.1% (FICI = 0.6). In combination B, 21 out of the 23 isolates studied (91.3%) showed synergism (FICI = 0.31–0.62). In conclusion, adaptation of the two-dimensional chequerboard assay is a reliable method to study in vitro three-drug combinations. Both three-drug combinations tested may be useful against drug-resistant isolates, although the combination including ofloxacin showed better efficacy, being of potential use in drug-susceptible and isoniazid-resistant isolates.