With the TRIS experiment we have performed absolute measurements of the sky brightness in a sky circle at $delta = +42^{circ}$ at the frequencies $ u =$ 0.60, 0.82 and 2.5 GHz. In this paper we discuss the techniques used to separate the different contributions to the sky emission and give an evaluation of the absolute temperature of the Cosmic Microwave Background. For the black-body temperature of the CMB we get: $T_{cmb}^{th}=(2.837 pm 0.129 pm 0.066)K$ at $ u=0.60$ GHz; $T_{cmb}^{th}=(2.803 pm 0.051 ^{+0.430} _{-0.300})K$ at $ u=0.82$ GHz; $T_{cmb}^{th}=(2.516 pm 0.139 pm 0.284)K$ at $ u=2.5$ GHz. The first error bar is statistic (1$sigma$) while the second one is systematic. These results represent a significant improvement with respect to the previous measurements. We have also set new limits to the free-free distortions, $ -6.3 times 10^{-6} < Y_{ff} < 12.6 times 10^{-6}$, and slightly improved the Bose-Einstein upper limit, $|mu| < 6 times 10^{-5}$, both at 95% confidence level.
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