So, Using your conversion chart(if you have one) you'd see 1 liter makes 1000 milliliters. So, if you have half of a liter(0.5), then you have 500 milliliters. If he adds 300 more, then you get 800 milliliters total, which is also equal to 0.8 liters. Hope this helps!
Let the 3 numbers be x,x+1,x+2
Given sum of those 3 consecutive integers are 78
X+x+1+x+2=78
3x+3=78
3(x+1)=78
X+1=26
X=26-1
X=25
Therefore the numbers are 25,26,27
