planning trip to london,paris, venice&rome

Are you too late: Nope. You're never too late as long as you can get a flight and hotel rooms that you are willing to pay for. As it does not seem that you are limited to the exact hotels that you wish to be staying at, you should be fine as long as the airfare works out.

Are three days enough for Paris: Depending on what you want to do, sure. Do you want to see all of Paris? If so, then three days are not enough. If you want an overview, three nights are fine. I might recommend two nights in Venice and take one more in Paris, though.

Between Venice and Rome you can either fly on an airline like EasyJet, or take the train for 4.5 hours.

For accommodations in Paris, I would recommend that you take a look at http://en.parisinfo.com as I find that they often have excellent rates for Paris hotels.

Don't be fooled. Ryan Air does not fly from Paris to Venice. Rather then using the close in airfields everyone else uses, they fly from Beauvais, a minor airfield over an hour from Paris, to a minor airfield, Treviso, another hour from Venice. Neither of these airfields has train service. You'll spend more time on buses than in the air.

Agree with Steven - reduce nights in Venice, and stay an extra night in another city of your choosing.

Also, be aware of Ryanair fees when comparing options, in particular boarding pass fee - unless you will be checking in online and have access to a computer printer in Paris, getting a boarding pass at Beauvais airport for your flight will cost you 40 EUR.

What about easy jet as they fly from paris CDG to Venice Marco Polo. I think they are better than ryan air too.

Now in terms of how many nights in Paris, I might be the wrong person to ask but I find I never have enough time in Paris. But I am very passionate about Paris. It depends on what you want to see.

Can not comment on Rome as I have never been there yet or Venice. We will be in Venice for 4 nights in october for the first time. I chose more nights in Venice because I want to roam around and get lost.
Otherwise I would say 3 is fine.

I think given the choice I would cut venice down to 2 nights and increase Paris to 4. Or dare I say this cut Venice out all together and do 5 in Paris. what about cutting rome all together? Fly London to Venice, Fly Venice to Paris and home from Paris. Increase London to 5 nights, keep Venice as 3 nights and then increase paris to 6 nights

I guess it is a matter of what your priorities are.

Guys thank you for your comments so far.
I agree, trip to venice should be down to 2 nights.
I am thinking about taking venice out and maybe spend an extra night in Paris and extra night in Rome.
I guess I can use that extra day to visit Florence and pisa.

So maybe I can fly easyjet from Paris to Pisa.
Visit the leaning tower, take a train to florence, spend a night there and then take a train to Rome.