Instead of three 40w light bulbs, can I use only two 43w halogen light bulbs in a ceiling light fixture with 3 lamp sockets rated at 40w maximum per socket?
Each socket (and the wire associated with it) is rated for its maximum wattage. While the overall wattage in the fixture may be less than the maximum allowed, the wattage in two of the sockets will be over, although only by about 8%. Most equipment is fairly conservatively rated, but it will not be compliant. And halogens do run hot.
Have you considered using LED bulbs instead. 60 watt equivalents use about 10 watts each. The price is dropping to less than $8 a bulb and they are rated to last for decades. Savings in electricity will probably pay for them in a little more than a year.