Can I claim a Tesla Model 3 under CCA class 54 for self employment use? Will I get 100% write off in first year? Can I also claim the HST credit for the purchase of it?

Yes, you can claim Tesla model 3 under class 54. From class 54 you can write off up to $55000 plus sales tax for the first year. By purchasing a Zero-emission vehicle you will receive a $5,000  incentive. Second, For shorter range plug-in hybrids, expect to receive a $2,500 incentive. 

For more information visit these sites: https://turbotax.intuit.ca/tips/zero-emission-vehicles-10875

https://www.tesla.com/en_CA/support/incentives