Used to teach year 3 and I would say that it depends entirely on the school and individual teachers. Most now have lots of liaison between year groups so big jumps are not so marked.
I'd say if they move from an infant school to a junior then difference in style is much more obvious but more in terms of responsibility and behaviour expectations rather than level of work difficuly....I'm sure someone will disagree totally but that's what I've found.
From what I recall the big change at my kids' school was....no home corner! In Y2 when they had finished their work they could go and play - I think in Y3 they could read a book if they wanted, but no floor cushions or toys.
They also started using proper exercise books with lines; in Y2 they still had those big books with plain paper.
I suspect it varies a lot from school to school, as sunnyside says. Mine never had any homework to speak of apart from spellings and reading (and reading homework ended once they were choosing their own books).
Lol MaloryTowers, I also taught Y3 and that was the biggest thing I noticed too. Some didn't even go onto the next line because I hadn't told them they could!
So, in answer to your question Charliecat, I'd say a main difference is that they're expected to be a lot more independent.
Also, many schools drop afternoon break in Y3, or don't build in 'free' time to the curriculum.
Teaching is also more structured IMO (not suggesting teaching in infants is poor, just that junior curr. demands it to be more rigid.)