  In the current framework, the standard parametrization of our Universe is the
so-called Lambda Cold Dark Matter ({\Lambda}CDM) model. Recently, Risaliti &
Lusso (2019) have shown a ~4{\sigma} tension with the {\Lambda}CDM model
through a model-independent parametrization of a Hubble Diagram of supernovae
Ia (SNe Ia) from the JLA survey and quasars. Model-independent approaches and
independent samples over a wide redshift range are key to testing this tension
and any possible systematics. Here we present an analysis of a combined Hubble
Diagram of SNe Ia, quasars, and gamma-ray bursts (GRBs) to check the agreement
of the quasar and GRB cosmological parameters at high redshifts (z>2) and to
test the concordance flat {\Lambda}CDM model with improved statistical
accuracy. We build a Hubble diagram with SNe Ia from the Pantheon sample
(Scolnic et al. 2018), quasars from the Risaliti & Lusso (2019) sample, and
GRBs from the Demianski et al. (2017a) sample, where quasars are standardised
through the observed non-linear relation between their ultraviolet and X-ray
emission and GRBs through the correlation between the spectral peak energy and
the isotropic-equivalent radiated energy (the so-called "Amati relation"). We
fit the data with cosmographic models consisting of a fourth-order logarithmic
polynomial and a fifth-order linear polynomial, and compare the results with
the expectations from a flat {\Lambda}CDM model. We confirm the tension between
the best fit cosmographic parameters and the {\Lambda}CDM model at ~4{\sigma}
with SNe Ia and quasars, at ~2{\sigma} with SNe Ia and GRBs, and at >4{\sigma}
with the whole SNe Ia+quasars+GRB data set. The completely independent
high-redshift Hubble diagrams of quasars and GRBs are fully consistent with
each other, strongly suggesting that the deviation from the standard model is
not due to unknown systematic effects but to new physics.
