[Merlin Series 3] filming in Pierrefonds in April
Thanks to the official Pierrefonds Website we already know when the Merlin crew'll be back in Pierrefonds in April 2010 - filming block 1 (i.e. stuff for earlier episodes):
From 1 to April 21 2010, filming takes place at the castle of Pierrefonds. Some areas may be temporarily closed/not accessable.
*FANGIRL \0/ SQUEE*
ETA: If the filming schedule 2010 is similar to those of 2008 and 2009 they'll be back in France to film block 2 in June and in September to film block 3. But we won't get any confirmation about such dates yet, in general the website updates 4-6 weeks prior to the actual filming, so we won't know about a very likely June date before the end of April/early May whereas a September date might be updated at the end of July.
NOTE: All this is just guessing (although VERY LIKELY), based on empirical values from 2009 - it's NOT 100% sure yet that they'll be in France in June and September!