I had a slice wound down my chest 5 inches long and also one to left arm 3 inches and to right arm 3 inches very nasty why can't I get a sick note?

Cause of wounds? If you want a past note doctors can only document time you were in treatment. If you are looking for time off the treated wounds probably arnt bad enough to justify time off.
You have to convince. Your dr that your injuries would justify you to rest at home. Best wishes!