I hope the question is not too silly, but I think it may help others too.

I want to thank the professors that are writing reference letters to me. I thought I'd put this at the end of my email requesting the aforementioned letters:

1. Thank you for taking the time to write this letter.
2. Thank you for taking the time and effort in writing this letter.
3. Thank you for dedicating your time and energy into this.

Are these options acceptable? What would be a more natural way of saying this stressing not only the time they took to write it, but also the pains they took, having to lose time writing this?

I think this question has the benefit of being useful in other cases. I just realized I don't really know how to thank people for something like this formally in English.

Related questions:
How to say thank you for the ....?
'Thank you for taking your time writing' or 'Thank you for taking your time to write' ?
Should I say "Thank you for your kindly attention" or "Thank you for you kind attention"

I hope the question is not too silly, but I think it may help others too.

I want to thank the professors that are writing reference letters to me. I thought I'd put this at the end of my email requesting the aforementioned letters:

1. Thank you for taking the time to write this letter.
2. Thank you for taking the time and effort in writing this letter.
3. Thank you for dedicating your time and energy into this.

Are these options acceptable? What would be a more natural way of saying this stressing not only the time they took to write it, but also the pains they took, having to lose time writing this?

I think this question has the benefit of being useful in other cases. I just realized I don't really know how to thank people for something like this formally in English.

Related questions:
How to say thank you for the ....?
'Thank you for taking your time writing' or 'Thank you for taking your time to write' ?
Should I say "Thank you for your kindly attention" or "Thank you for you kind attention"

EDIT: I should have emphasized this better, but I felt it was very important to thank the referees for the effort (not only the time spent) put into writing the letter. So I wanted that the answer to this question could highlight this desire. And thank you for your answers! All of them are excellent, it's a shame I can pick only one as the correct answer.