You can use a Atlas Soundolier AF 120 auto-transformer. OR
here is an example of a real professional autoformer
speaker selector which will run 6 pairs of speakers
off one 140 watt amplifier, by dividing the power.
this is not cheap but is cheaper than buying three
amplifiers: http/www.aecinfo.com/1/resourcefile/00/22/77/atint04.htm
do not confuse this with the cheap speaker selectors,
which are garbage.