How much does having published in high IF journals as an undergrad counts in the masters admissions process? I expect to have at least 3 publications (2 as co-author and 1 as first author) at respected venues by the time I apply to grad school (Mechanical Engineering) and want to know how much can that impact positively my application to top schools (MIT, Princeton, Cornell etc).
This can only be answered by the faculty and admissions committee of the institutions you actually apply to. It depends on how they see the relevance of your work and also on the competition you face from other candidates. 

However, it certainly isn't a negative factor. I'm not in the same field, and so don't know how common it is for undergrads to produce such papers. I suspect it is pretty rare. In this case, since you have an unusually positive application it would help you a lot - especially for a doctoral level application. How much it helps for a masters level depends on how research oriented the particular MS is. 

But the only way to know is to make the application. But note that these publications won't be the only thing that affects your chances.