Hi there!
Simple conversion will greatly help us here. Below I'll list some common conversions for pounds and ounces.
16 ounces = 1 pound
32 ounces = 2 pounds
These will greatly aid us in finding how many ounces are in 3lb and 3 ounces. If we know that 2 pounds = 32 ounces then we know that by multiplying that by 4 we'll get how many ounces 8lb is.
32 ounces * 4 = 128 ounces.
However, we're not done there! There are also a leftover 3 ounces meaning once we add that to our 128 ounces we'll get our final answer.
3 ounces + 128 ounces = 8lb and 3 ounces.
131 ounces = 8lb and 3 ounces.
And that's our final answer. :)