we are going to visit milan, venice and rome and what trains should we take from milan to venice and venice to rome the following day?
Go to www.trenitalia.com for your answers.
You may get more and better answers to your questions about Italian trains in the Italy Forum. You have posted in the Netherlands Forum.
Trenitalia.com gives you the schedules to mid-June.
When using this site, don't forget to use the Italian originals for the place names.
For the high-speed »Frecce« trains advance buying (credit card, home printing) gives you very considerable discount. Those tickets are only valid for the train on that specific day and hour. Advance buying can be done 2 to 3 months in advance.