tweak

int numCol = 10, numRow = 5;
float colSize, rowSize;

void setup() {
  size(500, 500);
  background(255);
  frameRate(3);
  colSize = width/numCol;
  rowSize = height/numRow;
}

void draw() {

  //float randColor1 = random(100, 200); -> not needed since you can simple add random() later in your fill() function?
  //float randColor2 = random(100, 200);
  //float randColor3 = random(100, 200);

  for (int i=0; i<numCol; i++) {
    //fill(random(100, 200), randColor1, randColor3, 100); //why would you want to do this here?
    for (int j=0; j<numRow; j++) {
      fill(random(100, 200), random(50, 150), random(100, 200), 100); //why use you randColors here instead of simply using random()?
      //rect((width/numCol) * i, (height/numRow) *j, (width/numCol) * i, (height/numRow) * j);
      rect(colSize*i, rowSize*j, colSize, rowSize); // the 3rd and 4th argument only need to be the size of one rectangle! your code created huge rectangles (that were hidden by the screen being too small), I think that also explains the offset in some way ... I'm not sure why though ... my rewrite starts at 0,0
    }
  }
}