While all 3 are made of cotton, they differ in how porous they are. A towel and tee shirt are probably a lot better at soaking up liquids (and therefore stains) than jeans are. I would suspect that jeans hide stains easier than towels and tee shirts do. Hope that helps.
The density of the material does determine how well the material absorbs a stain. The tighter the weave of the fabric, the closer the strands of cotton, and usually the more dense the material is. The colors we see in stains are also a consideration: each color has a different sized molecule. The larger the molecule, the less likely it will get inside the fabric strands and stain. Since jeans have a tight weave (and are dark in color, so they also hide the stain easier), they are less likely to absorb the larger colored molecules making them less likely to stain. A quick aside: any natural color (fruit juice, tomato stains, etc) will break down into a colorless compound in sunlight. Stain fighters work on most stuff, but for vegetable stains, sunlight works best.