What's my chance of getting into a good math or statistics graduate program, if I don't have impressive grades the first three semesters of college I'm a sophomore majoring in math and statistics at a large state university who has top 30 math programs. I really want to go to a very good math graduate program. However, my first three semester is full of Bs, and overall GPA is 3.5. And I happened to have a C and W on my transcript. My major GPA is pretty good though, a 3.98. But I'm still afraid that first year and half is going to hurt my chance of going to graduate school really bad. What do you guys think? Do you know where I stand? Anything I can do to counterbalance my unfavorable GPA? Thanks.
You major GPA of 3.98 is what counts most. "Full of B's" and overall GPA is 3.5 means "half A's, half B's," which is not at all bad. The one C and one W aren't great, but don't necessarily hurt you much. "Everyone" knows that when you take courses outside your major for "balance," you will likely get "worse" (and a few bad) grades. That's especially true the first three semesters. 

I'd expect your GPA to rise to about 3.7 as you take more courses closer to your major in the later years. As long as you "finish" on a strong note, that's (mostly) what counts.