We present an analysis of BeppoSAX observations of three clusters of galaxies which are amongst the most luminous in the Universe : RXJ1347-1145 , Zwicky 3146 and Abell 2390 .
Using data from both the Low Energy ( LECS ) and Medium Energy ( MECS ) Concentrator Spectrometers , and a joint analysis with the Phoswich Detection System ( PDS ) data above 10 keV , we constrain , with a relative uncertainty of between 7 and 42 per cent ( 90 per cent confidence level ) , the mean gas temperature in the three clusters .
These measurements are checked against any possible non-thermal contribution to the plasma emission and are shown to be robust .

We confirm that RXJ1347-1145 has a gas temperature that lies in the range between 13.2 and 22.3 keV at the 90 per cent confidence level , and is larger than 12.1 keV at 3 \sigma level .
The existence of such a hot galaxy cluster at redshift of about 0.45 implies an upper limit on the mean mass density in the Universe , \Omega _ { m } , of 0.5 .

Combining the BeppoSAX estimates for gas temperature and luminosity of the three clusters presented in this work with ASCA measurements available in the literature , we obtain a slope of 2.7 in the L - T relation once the physical properties are corrected from the contamination from the central cooling flows .