What can I do if my ex wife sent me receipts from the past three years (2011-2014)?

It depends on what the bills are for. Your judgment may provide you are responsible for a portion of certain expenses beyond child support. It may contain a provision limiting the time within which she has to submit bills to you, It may not limit the time. If there is no limit on when the bills can be submitted, then a three year period may (also may not) be reasonable. It also may depend on what the expenses are for, if you knew they were being incurred for example. As with many questions in this area, the questions, leads to many other questions. Look at your judgment.