Upvote:3
The UK asks about previous refusals. Obviously you should tell the truth. The US, Canada, and the UK exchange quite a lot of data and if they find that you lied your application will be automatically refused. When you tell truthfully that you have been refused, there will be extra scrutiny of your application.
You will have to be careful to explain that you are a genuine student, that you have good reasons to return to your home country after studying, and that you will not work illegally in the UK.
Since you have received several visa refusals, consider what impression you are giving in your applications.