Let’s look at the use of above mentioned prepositions in the context of time. All are used to denote time but there is a difference.
At This is used for indicating the time of the day .For instance at 5PM, at 6’O’ clock etc.
I got up at 6 am today
She normally leaves for work at 10 ‘o’ clock in the morning.
On This is used for indicating day and date. For example on Monday, on 7th June etc.
India got independence on 15th August 1947
The result of the general elections will be announced on 16th May.
In This is used for indicating the month and year. For example in January, In 2014 etc.
Usually, the Monsoon arrives in June every year.
The next Olympic games will be held in 2016