One can of sardines has >2x the omega 3s and is less likely to be counterfeit or adulterated as many suppliments have been found to be.
The vitamin D is another matter though, harder to get enough of in winter, but also easier to overdose on with suppliments.
I was under the impression that vitamin D is one of the harder supplements to overdose on and people don't take enough generally, is that not the case? (I don't take any).
Vitamin D dosage is a surprisingly controversial topic, with many voices claiming the recommended dose is much too low and you are very unlikely to overdose if you are not taking completely unreasonable amounts of it.
[1] https://www.fda.gov/food/metals-and-your-food/mercury-levels...
Mercury is actually not that big a deal if you are an adult male, or a woman who is done having kids. The main issue with it is developmental issues in children.
Unrelated: IMO, Sardines are the most delicious fish.