Context :

Aims : We combine measurements of weak gravitational lensing from the CFHTLS-Wide survey , supernovae Ia from CFHT SNLS and CMB anisotropies from WMAP5 to obtain joint constraints on cosmological parameters , in particular , the dark-energy equation-of-state parameter w .
We assess the influence of systematics in the data on the results and look for possible correlations with cosmological parameters .

Methods : We implemented an MCMC algorithm to sample the parameter space of a flat CDM model with a dark-energy component of constant w .
Systematics in the data are parametrised and included in the analysis .
We determine the influence of photometric calibration of SNIa data on cosmological results by calculating the response of the distance modulus to photometric zero-point variations .
The weak lensing data set is tested for anomalous field-to-field variations and a systematic shape measurement bias for high-redshift galaxies .

Results : Ignoring photometric uncertainties for SNLS biases cosmological parameters by at most 20 % of the statistical errors , using supernovae alone ; the parameter uncertainties are underestimated by 10 % .
The weak-lensing field-to-field variance between 1 deg ^ { 2 } -MegaCam pointings is 5 % -15 % higher than predicted from N -body simulations .
We find no bias in the lensing signal at high redshift , within the framework of a simple model , and marginalising over cosmological parameters .
Assuming a systematic underestimation of the lensing signal , the normalisation \sigma _ { 8 } increases by up to 8 % .
Combining all three probes we obtain -0.10 < 1 + w < 0.06 at 68 % confidence ( -0.18 < 1 + w < 0.12 at 95 % ) , including systematic errors .
Our results are therefore consistent with the cosmological constant \Lambda .
Systematics in the data increase the error bars by up to 35 % ; the best-fit values change by less than 0.15 \sigma .

Conclusions :