The simplest system is a common mix of collagen gel and cells in appropriate ratio (select it experimentally, when cells will feel themselves well). You can use 0.1-0.5% collagen (final concentration). Take stock solution of collagen in CH3COOH, add medium for cell cultivation for untill double final collagen concentration and achieve pH to neutral (or any other pH which your cells prefer). VERY Quickly: add equal volume of cell suspension, mix and aliquote into wells. Collagen matrix with cells is ready,
For preparing 3D collagen matrix you can opt methods like hydrogel preparation or freeze drying. Depending upon your application you can prefer the concentration of the collagen. Porogens can be used for increasing the porosity of scaffolds. Once matrix is ready you can UV sterilize it and after that pre soak it in medium. Add high density cells and monitor the cell proliferation.