I know it sounds obvious but it's worth repeating. An aircraft that you have previously flown 1 or 2 up will handle very differently with a few pie-eaters in the back.
As mentioned in a previous post, do the weight and balance calcs. that you haven't bothered with . .when only flying 2 up. Then one of the aforementioned pie-eaters might have to stay on the ground. It gives more confidence to the passengers to be refused entry rather than being told while taxiing back from an aborted take off "OK chunky it's next flight for you".
Also always but always check the actual fuel level in the tanks. Never accept "ah it's OK, just over half full" when your weight and balance shows you should have no more than half tanks. If you're on a long tarmac runway you might get away with it. Bumpy wet grass plus overweight no chance.