  We performed a detailed photometric analysis and photoionisation modelling on
three high excitation multiple shell planetary nebulae: NGC 3242, NGC 6826 and
NGC 7662. Archival HST/WFPC2 narrow band filter images were used to investigate
shocked regions by two independent methods: using low excitation ions (via H
alpha/[N II] vs H alpha/[S II] extended diagnostic diagrams) and by means of
high excitation species looking for regions of enhanced [O III]/H alpha line
ratios. Shocked region analysis via low excitation ions shows that major
deviations from the general inside to outside ionisation trend correspond only
to regions where FLIERs or LISs are located. The reduction on the SNR at the
outskirts of the [O III]/H alpha ratio maps, made us unable to unambiguously
identified for an enhancement on the [O III]/H alpha line ratio as an indicator
for shocks. For non-shocked regions we performed a photoionisation modelling
using Cloudy. Fittings to the [O III] and H alpha observed radial profiles lead
us to constrain on the free parameters of the density laws and filling factors
together to temperatures and luminosities for the CSPNe. Best fit models show a
very well representation of the [O III] and H alpha emission. Discrepancies in
the model fittings to the [N II] and [S II] profiles at NGC 6826 and NGC 3242,
can be attributed in the former case, due to a contamination by the light of
the CSPN and, in the latter case, either due to gas inhomogeneities within the
clumps or to a leaking of UV radiation.
