Number one does not sound natural. I would change it to: 1.) "Could you please provide your input on the e-mail below?" That sounds more natural. Regarding number two and three, they can pass as natural. You can also say: 2.) "Do you have any advice for me about going to the gym?" 3.) "Thank you for your insightful input."

"input" tends to sound cold and bureacratic, so perhaps more appropriate to technical responses in work email, but less appropriate for personal advice about exercise. although people can use it in casual context too, it's just less common if "could you give me advice about..." or "what do you think about..." could work instead. so if you do use it for less bureacratic context you want to make the rest of sentence sound more casual, such as asking to give or share input, as opposed to "provide" which is more formal/mechanistic/cold. (in 2nd sentence, "do you have input" is rather cold, as opposed to my suggestions above) for 1) the singular "input" is appropriate, not plural, because in this usage "sharing opinion or response" it's not considered necessary to count discrete thoughts separately even if multiple discrete statements are expected. i.e. "sharing your input" (singular) is speaking in general sense that includes sharing of multiple data points, and that applies even if it comes from multiple people, i.e. "we will gather input from the entire community". or from other angle, if you haven't yet finished explaining all your ideas, you can say "i haven't (yet) finished giving all my input (yet)" (singular). "inputs" (plural) is appropriate for discussing concrete or technical details, like describing an industrial process with 10 different material chemical "inputs" that are counted as separate objects, because they literally are separate objects.

1) Could you please provide your input on the below email? 2) Do you have any advice for me about going to the gym? 3) Thanks for your insightful input.