What is the best level to gamble +3x/20fcr magic circlets?

+3 Skill is ilvl 60
20FCR is ilvl 29

Items generated for the gamble stock are -5 through +4 levels of your character's level with a minimum item level of 5.

I'd recommend a level 65 so that every circlet is at least ilvl 60 ; they will be ilvl 60-69 and have the best chance of your desired roll.

Char level 62 if you gamble circlets/coronets,
char level 57 if you gamble coronets only.

60-65