This is all
speculation of course and I for one am happy with the deal I had just before Christmas (£300 in red); the only way I'd be miffed is if a new season T3 came out with everything the same but with better hubs(/wheels) and SPD pedals, which if it is heading down the price scale is highly unlikely IMHO.
Have a look at this hybrid bike which is sold in France (Edit: and UK as a Fitness 3)
http://www.decathlon.fr/btwin-forme-3-id_8188895.html : appears to be a similar if not identical white frame but with no carbon forks; you could see this translating into the new T3 easily, as it's rating in France is 3 out of 5 stars, it is foreseeable that this frame is left sitting in the warehouse ready to go at a lower price point. Also interestingly, currently the T3 in France goes for approx £34 more in value than in the UK when you look at the direct exchange rate conversion comparison (as a 'maths' check I did the comparison with the T5 and it works out almost exactly equal in cost £s v Euros with the same exercise, so perhaps the UK version arguably sits too low in the pricing ladder).
The T3 is showing as low on stock in the UK and is now positioned low down the internet page (it was head of the list before Christmas) - whilst the T5 is still showing high on stock for the few UK stores I looked at, including their latest store in Southampton that only opened just before Christmas; might be that the T3 in it's current position really is too good to be true and is due for down spec'ing / price point repositioning for the UK?