The reason I asked if you were paying for your own school is because the people I've known that paid for their own school tended to take it a little more seriously than the people who's parents paid. I'm NOT saying you don't take your schooling seriously, it's just a trend I've seen in other people.
A 3.0 isn't bad for some degrees, it all depends on what you want to do after college. If you want to go to grad school you'll want to bring that GPA up.
I'm naturally lazy myself so when I was in college I had to work EXTRA hard to make good grades. Smoking weed would before class would have made it much harder for me.
Here's what I used to do: I'd pack a fat bowl in my bong and set it on my dresser and I wouldn't touch it until AFTER I was done with ALL of my homework/studying for the night. I used weed as my motivator to make 4.0's a few semesters. I knew I wouldn't be able to concentrate if I tried to study high, so I just focused on my work knowing as soon as I was done with it I could smoke (and drink) the rest of the night.