Vit D is hard to go wrong with as long as it’s not some piddly strength/iu. Main rule is you should take it with a meal as it’s a fat-soluble vitamin* in order for it to be absorbed properly. It has to be D3 though not D2.
Omega 3… quality can vary greatly. I buy the liquid Nordic Oil (lemon flavoured, it’s fine) as it’s lab tested and has a high amount of EPA/DHA pers serving, but you can get that in caps. The big bodybuilding supplement places like MyProtein, Bulk etc do large bags of high strength Omega 3 caps. If you eat a decent amount of oily fish (e.g. a couple of servings a week) you’re probably fine and don’t need to supplement. The high street type stuff tends to be too low strength to be cost-effective. Ideally you want somewhere around 500mg combined EPA/DHA per serving.
*Vitamins are either fat or water soluble. This means you need the presence of fat or water for them to be bioavailable to the body. The former can be squirrelled away in your fat cells by your body to use as and when needed and so for Vit D for example you could take x7 doses once per week or x1 dose every day and the effect is the same. The latter are only absorbed by the body if they’re required which means you both need to hit your intakes of them daily but also that any excess is disposed of. So when you take a multivitamin and then wee neon green, it’s because your body doesn’t need all the Vit C or whatever and just lets it go in one end and come out the other.