  We analyze the occurrence frequency distributions of peak fluxes $P$, total
fluxes $E$, and durations $T$ of solar flares over the last three solar cycles
(during 1980--2010) from hard X-ray data of HXRBS/SMM, BATSE/CGRO, and RHESSI.
From the synthesized data we find powerlaw slopes with mean values of
$\alpha_P=1.72\pm0.08$ for the peak flux, $\alpha_E=1.60\pm0.14$ for the total
flux, and $\alpha_T=1.98\pm0.35$ for flare durations. We find a systematic
anti-correlation of the powerlaw slope of peak fluxes as a function of the
solar cycle, varying with an approximate sinusoidal variation
$\alpha_P(t)=\alpha_0+\Delta \alpha \cos{[2\pi (t-t_0)/T_{cycle}]}$, with a
mean of $\alpha_0=1.73$, a variation of $\Delta \alpha =0.14$, a solar cycle
period $T_{cycle}=12.6$ yrs, and a cycle minimum time $t_0=1984.1$. The
powerlaw slope is flattest during the maximum of a solar cycle, which indicates
a higher magnetic complexity of the solar corona that leads to an
overproportional rate of powerful flares.
