It is that simple. Like painting like van Gogh is simple: you just brush some paint on a canvas.
The simplification of the way we gain and lose weight is at the heart of many people's problems with dieting.
The problem with this simplification is that it's very difficult to calculate how many kcal are expended, and it can change at the drop of a hat, so your workout may over perform one day, and underperform the next.
OP, 3lb on the scales might go on overnight and come off overnight, without any exercise or change in diet. Scales aren't 100% accurate, water levels in the body change, the dinner you ate last night weighs something as does what you put into the toilet this morning (did you weight those? They're variables, so if you're needing to be as scientific as 3lb accuracy, you ought to have done) etc etc. If you've made a change for the better regarding your diet and exercise, you'll be healthier. The more muscle you build, the more you elevate your kcal expenditure. Keep going. Take photos. Make a graph and put this weight measurement on it. Keep going. There will be ups and downs, and, over time, a general downward trend in your weight, and increased tone in your appearance. Your 3lb is the equivalent of 'I thought I could walk, but today I tripped. Is this normal??' It's just not a biggie.