Question

If 3/4 gallon of paint covers 1/3 of the wall then how much paint is needed for entire wall

Answer 1

3/4 times 3
3 times 3 =9 
9/4
9 divided by 4 = 2.25
because you have to paint the whole wall you have to have 2.25 so you can't round down therefore you have to round up
so the answer would be 
3 gallons

Answer 2

I believe it would be $\frac{3}{4} + \frac{3}{4}+ \frac{3}{4}$ since you need to cover the wall three times in order to have it fully painted. So the answer should be $\frac{9}{4}$ which should round up to 3 gallons yes (CharlesTheDillPickle is correct on that).