Different cancers respond differently to glucose levels in the blood. The one I had fed on blood glucose, so ketosis was healthy for me. Some cancers prefer ketones. I would suggest doing some studying before deciding fasting is definitely a good idea. Either way, good luck.
It sounds like you are off to a great start.
I was recently diagnosed with ovarian cancer and am reading a lot of books! You didn’t ask, but a few books I’d recommend that discuss fasting/low carb/keto and cancer:
The Cancer Code by Jason Fung; Tripping Over the Truth, by David Serven-Shreiber; Anti-Cancer, by Travis Christofferson; and Hacking Chemo by Martha Tettenborn. Martha gives details on how she fasted before and after each chemo treatment and how that positively affected her cancer.
Praying it goes well for you!
You might be interested in this article about keto as diet during traditional treatments for cancer. There are also lectures on YouTube by some interesting researchers.
https://pubmed.ncbi.nlm.nih.gov/31399389/
Some cancers thrive off ketones. Not sure about testicular so maybe double check.
But also, 3 days will barely get you into deep atophogy. For something as serious as a cancer diagnosis, I would first double check my tumors preferred fuel source. And if it was glucose, I’d probably launch into like a 14-30 day fast, under the supervision of a doctor.
Good luck. I hope things go well for you! Give us an update sometime if you feel up to it.