Assuming you are you a lawful permanent resident of the U.S. and have an alien registration card (Green Card) then you are eligible for an eTA and:
Most applicants get approved within minutes. However, some applications can take several days to process so don’t wait until the last minute.
I suspect transit visas to be much quicker to process than even a regular visitor visa. If you’re in the US, a visitor visa takes on average 28 to 41 days (reference), so with 3 months to spare I think you’re on the safe side. I do suggest you apply at the LA office as it seems to be speedier than the NY one.
Quoting the CIC website:
Most applications for transit visas are processed within a few weeks
or less. Processing times vary depending on the visa office.
Credit:stackoverflow.com‘
4 Mar, 2024
4 Mar, 2024
5 Mar, 2024