For long term improvement, you need to be lifting and doing some long runs, but neither will help you improve in less than three weeks. Don't do any long runs between now and the race.
Are you lifting now? If not, don't start now. Lifting will hinder your performance over the short term of less than 3 weeks.
If you are lifting, cut it back by 75% or cut it out entirely until after the race. Your legs need to be as fresh as possible to the handle the 500m (60sec stand) 300m workouts.
In the last six days, do nothing hard. The focus should be on getting your legs as fresh as possible before the race. If your coach has you doing very hard workouts, sandbag them. Do NOT leave your race on the workout track. Do some easy workouts, like 10x100m strides followed by 3x200m. The 200's should be done at goal pace... no faster. Take as much rest as you need. Focus on how strong you feel and how you could pick up the pace if you needed to. Burn that into your brain.
In the race, visualize that same feeling with 300m to go. Your brain has these wonderful mechanisms to govern extreme effort. They will start to kick in with all sorts of negative messages. But your brain is wrong. Focus on whatever motivates you for the last 300m. Hate. Form. Breathing. Winning.