Lets try it this way. 3 numbers sum is equal to 81, thus A+B+C = 81. Now lets go to the next portion of the question.
The second number is 4 times the first.
B = 4A
The third number is 3 more than the first
C = 3+A
since we have everything in terms of A lets solve for A.
A+ 4A+ (3+A) = 81
do some simple math to find A.
A+4A + 3 + A = 81.
6A + 3 = 81
6A = 81 -3
6A = 78
A = 13
Now we got A lets solve for B and C.
B = 4A
B = 4(13)
B = 52
C = 3+A
C = 3+13
C = 16
Now lets check our answer
A+B+C = 81
13+52+16 = 81