There’s a wealth of material covering this route. It seems it’s become a very popular outing.
The master of this is Donald Hirsch, who has done the entire London-Paris route in 24 hours. More power to him! Others have fleshed out his route with maps, guides and descriptions, such as this one: Cycling from London to Paris in less than 24 hours.
At bikely.com, Here’s London to Paris section from Dieppe via Avenue Verte and forest tracks into Paris described by Donald Hirsch. The distance is 208km.
From this route it looks like the hotel we’ll be staying at the end of Day 2 is just less than 135km from Dieppe. So, all things being equal and with a grain of salt for actual conditions matching Google data, that makes Day 3 about 75km.
Here’s the guide by Mr Hirsch himself.
(And here’s our local copy of the same guide.)