Delete the "at" and you're fine.
Dear teachers...
Could you proofread the following sentence?
He goes abroad (at) every three months.
(meaning the interval between the trips is 3 months)
Many thanks
Delete the "at" and you're fine.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.