Complete a basic mix of your track, including all instruments and the lead vocal. Set the volume of the backing vocals so that they are audible in the mix, but quieter than your lead vocal track. Cut the lowest, most bass-filled frequencies from the backing vocal tracks, using a high-pass filter set to about 150Hz. If you do not own a high-pass filter, use a channel EQ plug-in to reduce the low end under 150Hz on each backing vocal track by around 6dB. Cut out frequencies below 50Hz entirely.
Listen to the overall mix, assessing the main vocal with a spectrum analyzer, if possible, to decide where in the EQ spectrum the lead vocal should be allowed to dominate. Cut the EQ of your backing vocals by as much as 6dB in this frequency range.
Listen to the mix again to assess the backing vocal. Consider further EQ reductions, especially to lower frequencies, where the bass instruments -- and possibly the lead vocal -- should dominate. Check the level of your backing vocals and adjust the volume of each track if necessary.