I had inguinal hernia surgery 3 weeks ago now wearing underwear is necessary or optional. if necessary should wear from now or after somedays is ok?

Up to you: It is up to you to wear it or not , not related to your 3 wks your hernia surgery done 3 wks ago , however wearing a underwear is a hygienic thing

As tolerated: All activity, including underwear wearing, should generally be done as tolerated by pain. One simple rule of thumb is: if it hurts, ease up. As there is always clinical variation, feel free to check with your surgeon as well, especially if you think something is wrong. Get well soon!