  Aim of this work is to determine C, N, and O abundances in stars of Galactic
open clusters of the Gaia-ESO survey and to compare the observed abundances
with those predicted by current stellar and Galactic evolution models. In this
pilot paper, we investigate the first three intermediate-age open clusters.
High-resolution spectra, observed with the FLAMES-UVES spectrograph on the ESO
VLT telescope, were analysed using a differential model atmosphere method.
Abundances of carbon were derived using the C2 band heads at 5135 and 5635.5
{\AA}. The wavelength interval 6470- 6490 {\AA}, with CN features, was analysed
to determine nitrogen abundances. Oxygen abundances were determined from the [O
i] line at 6300 {\AA}. The mean values of the elemental abundances in Trumpler
20 as determined from 42 stars are: [Fe/H] = 0.10 +- 0.08 (s.d.), [C/H] = -0.10
+- 0.07, [N/H] = 0.50 +- 0.07, and consequently C/N = 0.98 +- 0.12. We measure
from five giants in NGC 4815: [Fe/H] = -0.01 +- 0.04, [C/H] = -0.17 +- 0.08,
[N/H] = 0.53 +- 0.07, [O/H] = 0.12 +- 0.09, and C/N = 0.79 +- 0.08. We obtain
from 27 giants in NGC 6705: [Fe/H] = 0.0 +- 0.05, [C/H] = -0.08 +- 0.06, [N/H]
= 0.61 +- 0.07, [O/H] = 0.13 +- 0.05, and C/N = 0.83 +- 0.19. The C/N ratios of
stars in the investigated open clusters were compared with the ratios predicted
by stellar evolutionary models. For the corresponding stellar turn-off masses
from 1.9 to 3.3 solar masses, the observed C/N ratio values are very close to
the predictions of standard first dredge-up models as well as to models of
thermohaline extra-mixing. The average [O/H] abundance ratios of NGC 4815 and
NGC 6705 are compared with the predictions of two Galactic chemical evolution
models. The data are consistent with the evolution at the solar radius within
the errors.
