Breakfast is whenever you get up. During the week the vast majority of people have to be at work for 9am so obviously it will be before that, I don't probably around 7:30/8am for most people.
If you are at work, or at school then you have lunch during the lunch hour (normally from 12 until 1, or from 1 until 2, although some jobs might have greater flexibility with this). Normally it's just a sandwich or something.
And then dinner in the evening, which is the largest meal of the day. A lot of jobs finish at 5 o'clock in the evening so dinner would be around 6:30/7pm I think for a lot of people. I normally eat at around 7pm.
People snack during the day, it's not just three main meals and nothing else.
On Sundays some people have 'Sunday lunch', which is in the mid-afternoon, around 2pm/3pm. This is a large meal, with roasted meat (beef or chicken normally) and lots of vegetables, Yorkhshire puddings etc. It used to be that people had this every Sunday, as a tradition but I think that is less common nowadays. My mum does one maybe every 6 weeks or every 2 months, because it is quite a lot of effort to do on a weekly basis.