Hi, Tina. Nice to meet you. Hope we can have good chats together. And maybe we will meet in a party. Actually, I do my best to have new real friends, not just a list of name in my address book. Hope to talk to you soon. PLA
I have travelled to those areas 5 years ago and am happy to hear you chose to go. Paris has no shortage of accomodation of every level. What is your budget? Hosteleries would be your cheapest option. Google Paris hostels and you'll find plenty in almost any district you choose.
The same goes with Rome but much less to choose from. I recommend you staying near the main train terminus station since there are several nearby. Venice however is tough since there aren't that many around. Try to avoid the Ostello de Venizia because it is segregated with a curfew, and is simply institutional.