I never understand the difference between these three words, and nobody has never been able to explain it to me. I searched about it on many books, and websites, but I just didn't get it. Can you please try to explain it to me? Thank you.
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
used in response to a statement that is regarded as doubtful or surprising
used when making a statement that adds to or strengthens a previous statement
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):
without question or doubt
to a great degree
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.
For example, 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.