It's extremely easy to get from 2a to the train station. There is no need to take a bus (you only need to do that from terminal 1). 3 hours is plenty of time, assuming you aren't arriving in the middle of the day. Even then it's fine, just will take a bit longer. I've rountinely landed, collected my luggage, cleared customs, and made it to the train station in 60-90 mins, though this is arriving on early flights (my preference). I'd budget 90-2hrs if you're arriving later. Still fine. I had 6 different (non-French speaking) people arrive in cdg and make it to the train station to visit me in the south of France last year, including my 66 year old mother who had never been abroad before. My point is, you'll be fine :)
Tips: getting from the plane to customs and luggage is easy--just follow all the people! I highly suggest you use the bathroom right before customs (under the stairs). Make sure you get in the right line at customs (non-EU). However. Pay attention if you're waved to a "new" line--they will often open up new lines for both EU and non-EU holders to speed things along--I've seen lots of tourists get confused and not cut to the shorter line bc they don't speak French. Despite the intimidating line at customs, you will move quickly!
I suggest you get one of the free luggage carts when you collect your bags. You don't actually need it to carry your bags, its to help clear your path through the airport :) people do not move out of your way. Once you get your bags, follow the signs for "SNCF" "grande lignes" or "gare". I'm quite positive these are also in English. You really can't mess up because the terminal is a straight line, so even if you went the wrong way, you'd hit the end and just have to turn around. Eventually you'll get to 2e and be above the train station--just use the escalators or elevators to go down to the right level. Once you're there, scope out where the platforms are (there are two sides), but don't stress if you don't see your train posted yet--it won't show up until about 20-30 before your departure time. Once your train shows up, head outside and then down the escalator/steps to your platform. Do NOT use the elevator bc you'll find that it will only go up and not down (you need an SNCF employee to put in a key to go down to platform level and it takes forever). Once on the platform, find the display that shows a diagram of your train and get to the part of the platform where your car should arrive. Be prepared that it's a mad rush of pushing to get on the train--don't be too polite or you won't have anywhere to put your luggage. Then find your seats, relax, and enjoy!