There are plenty of tickets available thru scalpers and guides no mater where you go! I would suggest for you to wait and start looking maybe 2-3 weeks before your trip.
** In Rome, If you are planning to go to the Vatican City, Vatican museum (Sistine chapel) and St. Peter's Basilica, Easter could be though. We came back last week from Italy (VFR), and I reserved our time at Vatican museum eight days before getting to Rome (last leg of our trip). We purchased 'Roma Pass' and most places such as colosseum, we literally walked right in (NO reservation), even though there were huge lines outside colosseum and tons of guides offering their services and fast entry with them. ** In Florence, the official Accademia Museum site showed it was sold out weeks in advance, however !!! , there were plenty of tickets available online, a little bit more (5 Euro), for any day and time thru other sites. If you want to climb the Duomo's dome, figure out what day of the week you are there, their reservation fills out fast specially on weekend. I would suggest you make your reservation 3-4 days in advance.
** In Venice, we bought tickets directly at the sights, and walked right in. No online reservation and no long lines.
Good luck with your planning and trip