Which contract did you use for your purchase? Was it As-Is?
If it is not an AS-IS sale, and you are using the NVAR regional sales contract, the seller is responsible for doing the repairs for termite/wood-destroying insects. So this is not something you would need to be concerned about.
For radon, did you have a radon contingency? If so, you can ask the seller to install a radon mitigation system, which is very effective in bringing the radon levels well below the "danger" line of 4.0.
Your question is cut off regarding the third issue you were mentioning in your question, so I am not able to address it.
Regarding a credit to cover cost of repairs - it is not usually done by % of sales price. It is usually done based on cost of repairs. So you would figure out the cost of the termite treatment (should have been part of the report), and the radon mitigation system (in the range of $2000 from what I understand) and the third issue. All of that added together is the amount of credit or price reduction you should request if the seller is not repairing.
Feel free to call me if you would like to discuss.