Hello Lilith,
Ordinary and normal are very similar and often interchangeable. However there is some small difference.
Ordinary can imply that something is plain or not special. However, normal suggest that something is same as usual. - on a normal day we just sit at home and watch Netflix. Nothing out of the ordinary happens.
It’s important to note the expression ‘out of the ordinary’. This means unusual
Conventional suggest that something is acceptable or traditional. - We had a very conventional wedding.
Hope this helps. Let me know if you have any questions.