Some funding agencies may invite you to respond to the reviewer's comments. In addition, the reviews from an unsuccessful submission will help you write a successful subsequent proposal. Some advice on writing your rebuttal can be found below:
Thoroughly and carefully read all the review comments, double check the timing and format limits of your rebuttal.
For each reviewer, break down the specific positives and negatives and look at which positives and negatives are repeated across multiple reviewers. Where multiple reviewers have highlighted a problem, then prioritise responding to those problems.
Avoid being defensive or argumentative. Use objective language in a way that is informative and non-confrontational. Take the feedback and say what you will change to address the issues (they may even suggest suitable actions in the reviews).
Where reviewers disagree with each other on specific issues, then use the reviews that agree with you as supporting material to rebut the ones that don't.
Where multiple reviewers have noted a positive, you should intersperse some recognition of that into your rebuttal.
You could respond thematically, or by reviewer, but beware that responding by reviewer could lead to a lot of repetition.
Have a colleague review your rebuttal.
Start your rebuttal letter by thanking the reviewers for their time, and by noting the biggest positive that you can take from the reviews e.g. "I want to thank the reviewers for their detailed feedback, and I am delighted that they all agreed that my proposal was timely and significant".
