I visited Italy this summer, I know Rome and Florence very well,
but not Venice... Roma- Via dei Condotti, Via del Corso, all near
Piazza di Spagna, if you're into designer labels or more high-end
things. Via del Corso has a wide range of nice boutiques (both
pricey, and less pricey) and restaurants. Firenze- Via di/dei
Tornabuoni (high-end stuff), offspurts of that street you can find
less expensive boutiques and shoppes. Sorry I couldn't be of help
with Venice