As of date, there is no way to set a Target CPA at the keyword level. Target CPA and Enhanced CPC are bid strategy types that won’t allow you to set a budget per keyword — at least at this point.
However, there might be a workaround for now. If you have 3 keywords in an Ad Group, I suggest you separate each keyword into separate Ad Groups. This would mean 1 keyword per Ad Group instead of 3 keywords in 1 Ad Group. This way you can set the Target CPA per keyword at the Ad Group level.
This might be cumbersome for Ad Groups having a lot of keywords though.