According to David Kuchta in his book in "The Three-Piece Suit and Modern Masculinity", "In 1666, King Charles II felt it necessary to reform Englishmen's dress by introducing a fashion that developed into the three-piece suit." Beau Brummel honed it further in the 19th century into the beginning stages of the modern three-piece suit. In order to have all the pieces fit together, it is necessary to fit each garment properly.
The waistcoat or vest
Fitting the waistcoat is challenging. The fit must be proper in order to help the proper look of the suit and look even without the jacket. The waistcoat should extend up from the natural waist and cover the waistband of the trousers all the way around. If the shirt extends below the waistcoat, it will look sloppy and unkempt; nor should it be too long. The top of the waistcoat should extend above the suit coats enough to be visible when the coat buttons are done, yet not too high to leave the tie visible or cover the shirt's collar points. Make sure that the waistcoat is fitted so that it follows the natural lines of your back, narrower at your waist then wider at the top of your vest. This can be done through tailoring or a buckled strap at the back.
The jacket of three-piece suits needs to be fitted properly as well. It will need to be large enough to fit over the waistcoat, but must not be too loose. The best way to get suit jackets that will fit is by getting measured. The measurements you will need will be that of your chest, your arms and your middle. Do not stick out your chest or suck in your belly to ensure the proper fit of the jacket. The sleeves should extend down to your wrists, yet allow at least 1/2 inch or less of the shirt's sleeves to extend below the sleeves of the jacket. Make sure also that the lapels of the jacket lie flat against your chest when it's buttoned.
The trousers need to be higher at the waist than a regular two-piece suit, due to the waistcoat. If the trousers end at your natural waist, you may run the risk of the waistband showing beneath the bottom of the waistcoat. When you wear a three-piece suit, it's a good idea to rely on braces rather than a belt as it will not bulge beneath the waistcoat. Make sure that the trousers fall to the tops of your shoes or break slightly. They should not puddle on the top of your shoes. Measure your inseam carefully to ensure this does not happen.