Hello, My query is, does my low 12th percentage hinder my eligibility and admission to top schools in US and Canada?And my undergrad score is 3-yr BSc(8gpa)?

If you’re talking about grades you earned before college, they don’t count in the least, nor will they even be requested. While low scores or grades might keep you out of programs, they won’t get you into them. Applications are reviewed holistically. 

Don't become fixated on "best universities" or "elite universities," either. If you can get into Stanford or CMU, that's fantastic, but if not, there are hundreds of master's programmes at universities across the US and Canada.

You can enrol in any of these programmes to gain access to the US job market (which is typically what students seek), and you can now work legally for almost three years without an H1-B sponsor (and then access to a special pool of H1-B spots). While successful students tend to come from more well-known programmes, anyone with an MS in Computer Science currently has a bright future (or related degree). Every institution will have resources to assist you in locating an internship or a job, including salary and placement data and a career services department. Regional comprehensive students—smaller universities without doctoral programs—have excelled in my experience.

Other factors that I believe will have a big impact include:

Even though it may be tedious for people who dislike research, a worldwide publication will be very beneficial.
A few unconventional ventures. Consider giving your projects some beautiful names and describing them appropriately on resumes.
LORs and SOPs The only item that is "fully" in your possession is this. Create as many draughts as necessary until you are happy. Make adjustments. Show your buddies this. Present it to the instructor of your communication subject. Please do not copy and paste it from the internet or another counsellor. Make sure your introduction and conclusion are both captivating. Instead of discussing what your family is doing, talk about your interests and skills.
After completing your profile, it is challenging and wise to narrow down the list of good colleges. Act prudently. Please don't apply to colleges you don't want to attend. Never forget that getting into at least one college—but not just any—is your goal.

Therefore, you don't need to worry about your 12th grade. Try to perform well in GRE, TOEFL, or IELTS, which will boost your chances.