Any suggestions for a vacation from US to London, Paris, and Rom

We are planning a 2-3 weeks vacation in August to three cities - London, Paris, and Rome from US. I would like to hear your suggestions on

1. Local sightseeing in the three cities - Joining local tour groups?

2. Time should be spent in each city - Is there a tour group that covers all three cities?

3. Local transportations

4. Inter-city transportations - train, bus, or airplane. We are traveling with US passports.

4. Safety in the cities, especially Rome.

5. Currency - cash or credit card

6. Hotels - location and price

7. Car rentals and driving in Europe with US driver license and insurance.

8. Taking Europe cruises to save time and troubles? 

Many thanks in advance.