I solved 3 questions in APRIL challenge but my rating increased from 3389.66 to 3518.84 . Why there is so little increase in rating even after solving 3 problems.

Take a look at this link

The rating doesnâ€™t depend on how many questions you have solved instead it depends on your rank. So if the contest was easy you are expected to solve more problems to increase your ratings.

Happy Coding

But others who solved fewer questions got a much better ratingâ€¦?

If you look at mine it got reduced. I scored 220 points. I had rating of 3200 odd and now it is reduced to 2900 odd. It is relative and calculated exactly according to the formula given there. If someone had rating of 1000 or something will get their rating increased much more as they are not expected to solve 3 problems. At least that is what I understood. Correct me if I am wrong