When x is greater than 2 and less than or equal to 4, y = 2m+3 (x-2);

When x is greater than 4, y = 2m+6+n (x-4);

2, because when x=8, y = 33 and when x=6, y = 21;

So there are: 33 = 2m+6+4n; And 2 1=2m+6+2n.

So m =1.5; n = 6；

So when x=3.5, y=2m+3*(3.5-2)=3+4.5=7.5.

If a user's water consumption in March is 3.5 tons, the water fee that the user needs to pay is 7.5 yuan.

3. If y does not exceed 24 yuan, then X is greater than 4.

Then it is 24=3+6+6x-24, which means x=6.5.

If a user wants to pay no more than 24 yuan in April, he can use up to 6 tons of water.