Investing in single stocks can be fun and rewarding and you may beat the market over the long-term, but you need to be careful. As others have said/implied, its dangerous to put too many eggs in one basket.
If you are new to this type of stuff, but you want to dabble in 'direct' investment, I'd say put 80% into some sort of index fund split and then the last 20% in 1-3 stocks that you like.
For my wife's IRA, I have her in 3 stocks:
MCD, DIS, XOM -- helps us both sleep better at night to have money in companies like this. Of course we both also have lots of money in index funds.
We own about 25 other direct stocks. I personally enjoy investing, and I get a big kick out of being an owner of a more specific company versus just owning the index. Some people are bored to death or just scared of picking stocks.
To each their own. Bottom line is don't panic and hold course with your original plan.
As to 'how much can I make' I think the 7% posted above over a long-term (10 year) horizon is roughly right. Last year the S&P 500 returned 30% though, so it depends. Even with 7%, that almost doubles your $3k in 10 years.