It sounds like you're wanting to play the single player campaign mode in these games, and yes, at the easiest difficulty sometimes these games practically play themselves. Battlefield 3's difficulty is on par with the Call of Duty series in this regard, and it features an Easy mode that shouldn't be too challenging if you're a single player FPS veteran.
Difficulty is subjective though, so your experience may vary. You might want to try picking up one of the older Battlefield games for cheap, and seeing if you like the series' style before you invest in the latest title. There are differences between the control schemes, the weapons, the friendly/enemy AI, etc, that some people might find to be a dealbreaker. I have staunch "Call of Duty" aficionados that I play with frequently who can't stand the Battlefield series, although to me they are pretty darn similar.
At higher difficulties in the campaign, and especially online, you'll have to invest some time before you can survive or play competitively. The learning curve is significant, and many of the other players online have generally been playing various iterations of the game for years. On top of this, these games are popular among people who have copious free time, so they tend to be full of a lot of people who are experts at many aspects of the game, or who have figured out superior strategies. I can't recommend the online component of a game like Battlefield 3 to someone who intends to play casually.
