Three days to find males ar too early to find any males. You will need to be patient to find, eventually, you will find. For us, the heat method is not perfect to fine any males. If it has any proble, you may need strains that tend produce lots of males.

We incubate 8 L4 hermaphrodites at 30 °C for 4 to 6 hours, then place overnight at 15 °C. Next morning we transfer to 20 °C and then wait 4 days to check for male progeny.

Thank you Shin Murakami and Lucas Mergan for the answers. I will also try and increase the incubation period to 6 hours (at 30 degrees).

Might be harsh but it works, 37°C for 30 m to 1h (no more than that).

Male production efficiency sometimes is just random - it depends on the genotype, not always in a predictable way - so the best way to go is to upscale, prepare many plates with L4s hermaphrodites, and perform heat shock (even by testing different protocols and temperatures). Once you have a decent amount of males, maintain them by crossing with L4 hermaphrodites. Consider freezing the males if you are planning to reuse them in the future.

I used to take 5-6 NGM seeded plates, put 20 - 40 late L4 hermaphrodites on each, and incubate for 4 hours at 33°C, then transfer them to 20℃. and after four or five days, I will pick males on these plates.