I understand that each religion’s respective books claim their existence but are there other sources or evidence that points to an actual person apart from the mythology as a real person? Any other sources that is widely accepted by historical scholars to be true?
More can always be said on this matter, especially on the historicity of Siddhartha Gautama, as that's ground so far little-trod on the sub beyond the thread I'm linking below. For the other two, questions about these do pop up often, so we can refer to the FAQ - though again, if anyone would like to have a go of their own, don't let this post stop you!
The Religion & Mythology section of the FAQ has several threads on the historicity of Jesus (I particularly commend the ones by u/talondearg), and one on the historicity of Muhammad. For the Buddha, u/WaylonWillie examines the case here.
And an important caveat: Whilst we can speak of religious figures in their historical context, we cannot do the same of any of their miracles or other matters of theological import, for various reasons (not least in that miracles don't fossilise). That's the bailiwick of the Theology Department, over in the other building.
There are answers regarding the historicity of Christ and Muhammad in the FAQ:
Here's the answer on Muhammad by /u/shlin28 and then there's 4 answers on Christ in this section of the FAQ by /u/talondearg, /u/jasoncaspian and /u/kookingpot
As for the Buddha, Reddit's search function did its job for once and turned up what seems like a solid answer by /u/artfulorpheus
https://www.reddit.com/r/AskHistorians/comments/7584vs/what_do_we_know_about_the_historicity_of_the/
Of course other answers are always great :)