You cannot apply for the I-765 before your K3 is granted.
If you live in Ontario I would strongly suggest NOT going for the K3. The processing time is about the same as the CR1, and the CR1 is a permanent resident visa and is work authorized. The K3 is NOT work authorzed, and will cost you a further $1010 after you arrive in the US to apply for adjustment of status and employment authorization.
For either of the K visas, as well as the CR1 visa, you will be waiting out the process in Canada, not the US. You can visit the US for short times, but you cannot stay there for the whole year it'll take the process to complete.
Once again, for marriage based information to the US see the website
http://www.visajourney.com