When I hear "add tests" it's always a red flag to me. Imho tests are not optional, they are implied, they are a tool to build your stuff (like scaffolding). If tests are added at the end, how would one know the code written before that was correct?
The later tests are added, the less of their value is leveraged.
IMO, where tests are adding most productivity is when they enable rapid refactoring. When you test with that goal in mind it doesn't really matter much where in the cycle tests are added.
Yes, it should not matter 3 or 30, once you start to test the higher level/API then some classes don't need a separate test, because they are tested when being invoked by other tested code.
The later tests are added, the less of their value is leveraged.