I'll outline the third option you neglected first. I assume $10,000 in cash to play with, 7% returns.
The choice becomes RRSP or taxable account. You can put the $10,000 into your taxable account, it will grow 7%/year and reach approximately $13,000 in 2021. In 2021 you sell and pay gains on $3000 which is $675, but since you put the entire amount into the RRSP, you actually receive a refund $675 larger than expected. Gains are taxed at 50% but when you put money into RRSP its at 100%. This scenario yields a refund of $5175 (the $4500 plus $675), meaning that initial $10,000 will be $18,175 inside your RRSP in 2021 assuming 7% gains.
Your scenario, you in and defer, means the same $10,000 goes in and grows to the same $13,000 (assume the same gains for fairness) but you get an additional $4500 in refund, bringing the total to $17,500 in 2021.
Third option is to put in $10,000, claim and receive $3500 immediately. In 2021 this is then worth $17,700 roughly the same as the second option as you noted in your OP.
The first option assumes lots of RRSP room. If the money doesn't grow, or shrinks, it would have done the same inside your RRSP but you can claim the loss when its on taxable accounts.