All three are good. I use Scottrade and Etrade. I know someone who has TD Ameritrade. With Scottrade, I like that they have a branch close by and the people there are friendly with me and even call me Mr. Last Name when I walk in...lol. If you are going to buy stocks that are under $1, then Scottrade is not the best bet, since they add on 1/2 percent to each trade. But other than that, they are great. Regular trades are $7.
Etrade is very good and you can buy and sell stocks that are under $1 and it will only cost you $9.99. One caveat is that since Etrade is also a market maker, it seems to take a bit longer when doing the trade. This is only seconds longer, but it seems that when I do at trade at Scottrade it is almost immediately confirmed, while Etrade might take 20 seconds.
So, you pay a little extra with stocks under $1 at Scottrade, but you might end up paying extra through Etrade, but not in a way that you can notice. Both are legit. As usual, use limit orders only when buying stocks. For stocks under $1 you must use limit orders, anyways, so you can't get scammed in that way.
Etrade is also a bank. You can apply for a checking account with them and rapidly transfer money back and forth as many times as you wish, and it is immediate. Say you have 3K in your checking at Etrade. You can buy a stock 10 seconds later....just transfer it over to your Etrade brokerage and fire away.
Both Scottrade and Etrade also have check writing and Visa/MC debit features that you can use through their brokerage account, as well. You can order checks and/or get a card sent in the mail, and when you sell a stock you can use the money immediately with the card or write a check against it. I have the Etrade checking and Visa debit, but I have not ordered the Etrade brokerage Visa Debit or checks (since I have Etrade checking), and I have not ordered the Scottrade checks or debit card, but I know they have it.
I think you would do fine with any of the big three you mentioned.