Jon P. answered 06/02/15
Knowledgeable Math, Science, SAT, ACT tutor - Harvard honors grad
For 3 tests to average to at least 90, the total of all the grades has to be at least 270. However, if the average was between 89.5 and 90, your teacher would probably round up. So let's say the total has to be at least 269, for an average of 89 2/3.
You've got 80 points so far, so the next two tests have to add up to 189. There are different ways to do that, such as an 89 and a 100, or a 94 and a 95. But assuming that 100 is the highest grade you can get on a test, then if you get an 88 or less on either of them, there's probably no way to do it.