is the lowest test mark for math 1ls3 dropped?
so only 3 test will count?

Yes, the lowest mark you get for the tests is dropped.

Yep, they only count 3 out of the 4. And I'm pretty sure they have it this year too, they do the best of the 2 calculations, either 60/40 or 10/90, and give you the better mark out of the two. Has your prof talked about that in class?