Let's first focus on their usages as adverbs. Adjective meanings are different.
A. As adverbs, 的确 and 确实 are the same. They mean 'indeed': without any question
For example, "A: 你会给朋友推荐这本书吗？B: 没错，我(的确／确实／真的)会给朋友推荐这本书。" （A: Would you recommend this book to your friends? B: Yes, I will indeed recommend this book to my friends.)
B. As an adverb, 实在 means really/very: — used to refer to what is true or intensive (the latter is not in 的确／确实):
For example, you can say "实在漂亮". It just means really/very beautiful.
In summary, to say 'in fact or indeed', you can use any of 的确, 确实 and 实在. But to mean 'really / very', you can only use 实在.
After watching the movie, I think it is indeed a good one. 看完电影以后，我觉这(的确／确实／实在)是部不错的电影。
After I won a free movie ticket, I was really really happy. 赢得了免费的电影票，我实在是太高兴了. (but not 的确／确实)
Hope it helps a bit. As adjectives, they are also different but less subtle.