Right, Tankslapper, Bull and I did this a couple of years back. Here's a few things...
The Ambleside YHA us awful and expensive. Boggle Hole is worse (and nowhere near a pub for the 50 beers you'll need when you get there). So I guess spend about £10 pppn more and try for nice B&Bs.
We left St Bees about 8am, and raced into Ambleside at about 4.30. To be honest, we could have at least got to Troutbeck that night, and would have been much better off for it, and day 2 is a MONSTER. I'd try to find somewhere there - getting over that one extra valley when you're stuck in the dismal hell that is Moses Dale above Shap will make you feel much better. It took is 2 hours of carrying to get out of there.
If you're doing it in 3 days, you really need to clear Swaledale completely and get beyond Richmond on day 2. We stayed in an excellent pub in a village called Danby Whiske that night. Last day is a bit easier than you'd think, but we both completely lost the plot at different points. I nearly got flattened by a caravan and would have gone to the length of riding to a bike shop and especially purchasing some Bombers to kill that driver; and Shane lost it when 'one last hill' turned out not to be so, about 4 miles from the finish!
Now then, you say you're carrying kit. We were the fittest we've ever been by soem margin, and struggled in three days. It's about 250 miles, and over 30,000 ft of climbing cumulatively. We were bloody glad of support, and you'll definitely need lights if you're leaving it as late as end of Sept. Our times were:
St Bees --> Ambleside 8.5 hours
Ambleside --> Danby Whiske. We didn't manage it, got scraped up off the road after 14 hours pretty solid. We insisted to be dropped off there the next day to carry on.
5 miles W of Richmond --> Robin Hood's 12.5 hours
Oh, and really, make sure you've got someone to push the bikes back up the hill from RHB beach 
