I have been wearing glasses for a few years only to see the board at school. my prescription is -3 in both eyes. I sometimes have to squint and strain to see things out in public places. Should I wear them all the time?

Also should i wear my glasses when going to have a haircut and keep them on for the duration of the haircut?

Yes, with that correction you need your glasses all the time. But no, you should remove them while they shampoo and cut your hair...put them back on afterwards.